the kola superdeep drillhole and the nature of seismic boundaries the kola superdeep drillhole was drilled through 7 km of proterozoic rocks of the pechenga structure and 5 km of the archean basement in the baltic shield to 12 km depth the results of drilling revealed a significant heterogeneity of physical properties of the crystalline rocks and a surprisingly high porosity and water content throughout the section seismic wide angle reflection data obtained in the region of the hole were used to delineate the volcanic sedimentary layers of the pechenga structure and the intersecting faults these studies revealed also a subhorizontal boundary at a depth of 7 km of uncertain origin core sampling implies that this boundary is associated with a change in core pressure with higher porosity and with velocity anisotropy a change in rock fissuring and in the fluid regime correlates with a velocity inversion zone reached by the hole at a depth of 4 5 km

catalogue of palaeomagnetic directions and poles from fennoscandia archaean to tertiary abstract palaeomagnetic data from fennoscandia ranging from the archaean to the tertiary have been compiled into a catalogue the data are presented in table format listing precambrian data according to tectonomagmatic blocks and late precambrian phanerozoic data according to geological periods each pole is graded with the modified briden duff classification scheme the catalogue complete to the end of 1988 contains 350 entries from 31 tectonomagmatic blocks and or geological periods normal and reversed polarity data are listed separately to allow polarity asymmetries to be studied each entry also has an indexed abstract summarizing relevant information such as the age of the rock the age of the natural remanent magnetization and the basis for the assigned reliability grade all the data are stored in the palaeomagnetic data bank which will be updated annually with new data the catalogue is the basic source of data for the microcomputer based palaeomagnetic database for fennoscandia now being compiled

superficial granitic layering in shield areas abstract a survey of structural studies suggests that superficial granitic layering is a feature of general occurrence in shield areas the top granitic layer exhibits both lower seismic wave velocities and above all a considerably lower quality factor than the underlying proper granite from which it is separated by a first order discontinuity at a depth of 1 2 km because of its fractured and brittle nature the superficial layer is both of geophysical and engineering significance not the least in connection with plans for nuclear waste disposal

crustal structure off norway 62 to 70 north abstract extensive geophysical surveys have been undertaken on the volcanic passive continental margin offshore norway between 62 and 70 n during the last 25 years three main margin segments have been identified the lofoten vesteralen margin the voring margin and the more margin the main features of the margins are prominent marginal highs including seaward dipping reflector sequences and an up to 22 km thick volcanic and transitional crust prominent escarpments the voring plateau escarpment and the faeroe shetland escarpment and up to 12 km deep post jurassic sedimentary basins east of the escarpments velocity depth solutions from about 250 sonobuoys expanding spread profiles and refraction profiles have been compiled and contoured isovelocity horizon contour maps and velocity transects outline a crust which broadly thickens from an oceanic crust with a normal oceanic type velocity structure to a ca 35 km thick continental crust with a continental velocity structure beneath the norwegian coast anomalous features include local crustal thickening below the more and vering marginal highs and high velocity bodies in the lower crust in the extension of the precambrian lofoten vesteralen archipelago the free air seasat derived gravity anomalies show a good correlation with the high velocity bodies and show prominent ne trending highs from the rockall plateau porcupine plateau region over the more voring and lofoten vesteralen margins to the southwestern barents sea

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

uplift of deep crust during orogenic extensional collapse a model based on field studies in the sogn sunnfjord region of western norway the western gneiss region wgr in norway experienced high pressure metamorphism during silurian devonian continent continent collision the eclogite bearing lower crust is separated from the middle and upper crust by major detachment zones formed during extensional collapse of the orogen formation of the devonian basins is related to the extension the footwall of the detachment zones comprises three structural and metamorphic zones the upper zone zone 1 is characterized by penetrative homogeneous down to the west simple shear developed under retrograde greenschist facies metamorphism zone 2 suffered inhomogeneous simple shear of the same polarity petrography and mineral chemistry data from the lower zone zone 3 show a record of initial eclogite facies metamorphism at 600 c and 16 kbar which was decompressed almost isothermally to amphibolite facies conditions at 550 c and 10 12 kbar both the eclogite and amphibolite facies metamorphism developed in a regime of pure shear with vertical shortening the rapid decompression records an approach of approximately 20 km to the surface related to uplift that was probably the result of the removal of a thickened thermal boundary layer in the mantle lithosphere the pure shear regime which developed initially in the lower crust was truncated by zones of simple shear as the lower crust was uplifted to middle and upper crustal levels extension by simple shear in the upper crust was rooted in the lower crust where extension occurred by pure shear the shear zones in zones 1 and 2 did not penetrate the pure shear regime of the lower crust a considerable amount of tectonic stripping of the orogenic welt predates deposition of the devonian sedimentary basins

the present rate of uplift of fennoscandia implies a low viscosity asthenosphere the rate of displacement in fennoscandia has been intensively discussed for many years it is now widely accepted to be an isostatic response of the glacial history of the area the earth s present response to deglaciation in fennoscandia is simulated using a three dimensional 3d viscoelastic model in which the asthenosphere and mantle viscosity are allowed to vary so that the maximum rate of present uplift matches its observed value the deglaciation history is considered to be known and the c14 datings are converted to sidereal years the pattern of the present uplift gives a firm match with the observed data when a low viscosity asthenosphere is introduced assuming a 15 000 years load cycle i e the glacier was applied to the surface for 15 000 years before the melting started the best fitting earth viscosity model is a 1024 nm lithosphere overlying a 75 km thick 2 0 1019 pas asthenosphere and a 1 2 1021 pas mantle the simulations suggest a remaining maximum uplift of 40 m

tomographic estimates of sub moho seismic velocities in fennoscandia and structural implications abstract pn and sn velocity estimates are derived for the sub moho structure beneath fennoscandia using a tomographic conjugate gradient scheme observational data stem from local earthquake recordings of crustal phases pg sg lg and the sub crustal phases pn and sn by the fennoscandian seismograph network unknowns are pn and sn velocities and event hypocenter parameters crustal thicknesses are assumed as known and are not estimated whereas velocities are allowed to vary the most prominent p and s velocity anomalies are found in the northwest inland lofoten and southwest more of norway and for shear waves also in central finland there is a good correlation between the velocity anomalies in norway and observed negative bouguer anomalies indicating the presence of low density sub moho material in these areas geologically the pn and sn velocity anomalies appear to be associated with geodynamic processes tied to the opening of the norwegian greenland sea

a model of the deep structure of the northeastern part of the baltic shield based on joint interpretation of seismic gravity magnetic and heat flow data abstract we have combined gravimetric geothermal and magnetic data with seismic results to characterize the crust of the northeastern part of the baltic shield the thickness of the crust varies between 31 and 48 km and four internal layers can be distinguished each with different physical properties and possibly with different geological origins a joint analysis of seismic and gravity data was used to construct maps of the moho relief and of the crust mantle transition layer our proposed one dimensional geophysical crustal model of the baltic shield may be considered as a basis for more detailed investigations

recording marine airgun shots at offsets between 300 and 700 km this paper demonstrates that under favorable conditions by using multichannel recording and subsequent stacking of adjacent records marine airgun shots have been detected at offset distances up

the south scandinavian crust structural complexities from seismic reflection and refraction profiling abstract pioneering work on mapping the scandinavian crust commenced in the early 1960s and since then numerous profiling surveys have been undertaken particularly as part of the on going eugeno s project however the most significant contribution to mapping crustal structural details came from the m v mobil search cruises in the skagerrak and off the west coast of norway 16 s twt reflection profiling all past and present crustal profiling results have been integrated to produce detailed maps of moho depths and crustal thicknesses for south scandinavia the thinnest crust is found in the north sea and skagerrak approximately 20 km while east central sweden features very thick crust approximately 50 km other interesting features are the apparent correlation between crustal thinning and sedimentation subsidence magmatic activity earthquake occurrences and the tectonic age of the crust moho depths and the crustal thicknesses clearly reflect the tectonic evolution and the present structural features of the region investigated

interpretation of the reflections in the siljan ring area based on results from the gravberg 1 borehole abstract during 1985 about 80 km of surface multichannel seismic reflection data were collected across the meteorite impact in the siljan ring area in central sweden the area consists mainly of granitic and gneissic rock ranging in age from 1400 to 2000 million years with remnants of palaeozoic sedimentary rocks preserved by downfaulting after a meteorite impact at 360 ma thereby forming a circular ring about 40 km in diameter dolerite intrusions ranging in age from 850 to 1700 million years are also present the seismic data revealed several high amplitude laterally continuous sub horizontal reflections in the northern part of the structure the high amplitude reflections and a possible intermediate low velocity zone were contributing factors in choosing the site for the gravberg 1 deep earth gas test well drilling and vertical seismic profiling vsp found that the reflectors were associated with dolerite sills which had intruded into the granite and which range in thickness from a few metres up to 60 m and with a pre impact area extent of at least 800 km 2 studies of amplitude and frequency versus offset afvo show the observations are compatible with a model of simple granite dolerite granite layering

crustal structure across the tornquist zone southwestern edge of the baltic shield a review of the eugeno s geophysical results abstract a seismic refraction and wide angle reflection experiment was carried out in the summer of 1984 in southern sweden denmark and northernmost germany the eugeno s project european geotraverse northern segment southern part was part of the european geotraverse observations were made along five profiles reaching a total length of 2100 km in addition to 51 explosions an airgun array was operated along the offshore parts of the profiles giving signals to distances greater than 200 km three profiles cross the southwestern edge of the baltic shield and two run along the edge one on each side the results show that in this area the moho discontinuity is a sharp boundary the crustal thicknesses vary significantly between 26 km and 47 km in some places the transition zones are narrow in others they are broad the variations are mostly in the upper crystalline layer and in the sediments the sedimentary thicknesses are between 0 and 10 km the general relationship is such that thick sediments correlate with thin upper crystalline crust and shallow moho depths the thickness of the lower crust approximately 20 km is quite uniform throughout the region of the edge of the baltic shield a well known gravity anomaly in denmark around silkeborg is explained by high density and high velocity material anomalously close to the bottom of the sediments a number of regional lineaments known from geological studies and reflection profiles can be followed through the crust into the upper mantle some pieces of shield crust have become detached and are now found in the ringkobing fyn basement high in the middle of denmark

the crustal structure along the polar profile from seismic refraction investigations abstract seismic refraction investigations along a 440 km long profije on the northern baltic shield have resolved the crustal structure in this area of archaean to early proterozoic lithosphere formation the profile called the polar profile extends approximately along a sw ne oriented line from the karelian province in northern finland across the lapland granulite belt and the kola peninsula province to the varanger peninsula in northeastern norway at six shotpoints large explosions 200 1680 kg and at three shotpoints small explosions 80 kg were detonated and recorded at an average station spacing of 2 km providing high quality record sections a two dimensional cross section of the crust was obtained by forward modelling using ray tracing techniques high velocity bodies are found in the upper crust related to the karasjok kittila greenstone belt and the lapland granulite belt they extend to a depth of 6 13 km in the karelian province in the southwest a low velocity zone was found between the depths of 8 and 14 km the middle crust shows a slight increase in the average velocities from the southwest to the northeast and a small velocity jump is found along a mid crustal boundary between 18 and 21 km the thickness of the middle crust varies between 16 and 18 km the lower crust and the crust mantle boundary moho show considerable lateral variation the top of the lower crust lies between 26 and 33 km while its thickness decreases from 21 km in the southwest to 10 14 km beneath the lapland granulite belt and the inari terrain reaching 20 km again in the extreme northeast the velocities also change laterally the thin lower crust is characterized by rather low velocities 6 8 6 9 km s whereas in the southwest and northeast the velocities 6 9 7 3 km s resemble more typical shield structures the moho is found at 47 km in the karelian province rises to 40 km beneath the lapland granulite belt and descends to 46 km in the northeastern part of the kola peninsula province the upper mantle velocities at the moho range from 8 1 km s in the region of the thin crust to 8 5 km s and more beneath the karelian province it is tempting to suggest that the anomalous lower crust underlying the lapland granulite belt and the inari terrain may represent the remnants of an early proterozoic back arc basin that was active prior to the 2 0 to 1 9 ga plate convergence event during which the lapland granulite belt was thrust onto the archaean basement of the karelian province another explanation is to assume that the velocity reduction in the anomalous lower crust was caused by a rather pronounced uplift of this region following the 1 9 ga collision event

on upper mantle seismic heterogeneities beneath fennoscandia abstract three dimensional seismic mapping of the upper mantle beneath fennoscandia baltic shield using an ach type of inversion technique in combination with p wave travel time residual observations from the local seismograph network gave the following results the central parts of the baltic shield are characterized by relatively high seismic velocities down to approximately 300 km those parts of the shield most affected by the caledonide orogeny exhibit relatively low velocities particularly in the uppermost 100 km depth interval the lower part of the upper mantle 300 600 km does not exhibit pronounced seismic velocity anomalies and in this respect is in contrast to results from similar studies in regions subjected to neotectonic processes like parts of central and southeastern europe the seismic anomaly pattern in the presumed thickened lithosphere is in quantitative agreement with similar ones derived from surface wave dispersion analysis and inversion of electrical measurements the general orientation of these anomalies coincides with that of the glacial uplift

evolution of the petrological and seismic moho implications for the continental crust mantle boundary the chemical and petrological composition of mafic rocks from the lower continental crust are discussed by comparing mafic granulites and meta gabbroic rocks from the ivrea zone and the northern hessian depression nhd xenolith suite both regions contain contrasting types of meta mafic lithologies i former basaltic rocks with trace element patterns ranging from morb iike to subduction related or intra plate tholeutes and u ca and al enriched plagiodase dominated gabbroic rocks showing positive eu anomalies generated by complex deep crustal magmatic processes such as fractionation accumulation of plagiodase and pyroxene and crustal contamination the absence of typical garnet omphadte parageneses in these rocks indicates that the eclogite stability field was not reached during palaeozoic orogenic processes a compilation of experimentally determined p wave velocities and densities for mafic granulites gabbroic rocks eclogites and peridotites is used to evaluate key physical properties of lower crustal mafic rocks during crystal thickening caused by continent continent collision in a step by step scenario it is demonstrated that the position of the seismic moho defined as a first order velocity discontinuity and the petrological moho defined as the boundary between non peridotitic crustal rocks and olivine dominated rocks is not identical for the case that mafic rocks are transformed into edogites at the base of orogenically thickened crust p wave velocities of eclogites largely overlap with those of peridotites although their densities are significantly higher than common upper mantle rocks as a consequence refraction seismic field studies may not detect edogites as crustal rocks this means that the seismic moho detected by refraction seismic field studies appears at the upper boundary between edogites and overlying crustal units since edogites generally have higher densities than peridotites they might be recycled into the deeper lithosphere thereby transferring excess eu into the upper mantle this process could be a due for understanding the negative euanomaly in the upper continental crust which is apparently not balanced quantitatively by the abundance of common mafic crustal rocks

seismic models of the lower lithosphere beneath the southern baltic sea between sweden and poland abstract based on recordings from the northern part of the baltic sea black sea profile and eugeno s profile 4 2 d seismic models have been constructed for a profile across the southwestern part of the east european platform from southern sweden to northern poland the thickness of the crust along the profile varies from 33 to 47 km the moho p wave velocity is 8 0 8 3 km s the uppermost mantle has a fine structure with alternating layers of higher and lower velocities down to a depth of about 120 km three alternating high and low velocity layers have been modelled the velocities vary between 8 2 and 8 7 km s in the high velocity layers and 8 0 and 8 5 km s in the low velocity layers a tendency of the depth of the moho and the depth of the uppermost mantle seismic boundaries to increase towards the southeast is observed

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

tomographic study of crustal velocity structure in southern finland abstract an inverse tomographic problem of the construction of a 3 d velocity structure has been solved for depths of 10 20 30 and 40 km using the p wave arrivals registered by the seismic array in southern finland and on the dss lines sveka 1981 and baltic 1982 the interpreted results verify and enlarge dss data obtained earlier for a high velocity structure of the crust and explain some small lateral velocity variations the concordant tomographic and dss data specify a 3 d model for a complex geological structure in the region

deep seismic transects across the sheared western barents sea svalbard continental margin abstract we have used recently acquired deep seismic reflection and refraction profiles together with a large amount of conventional mcs data to study the 1000 km long sheared western barents sea svalbard margin with the objective of determining its crustal structure and tectonic history the margin developed from a megashear zone which linked the norwegian greenland sea and the eurasia basin during the eocene opening the margin is composed of two large shear segments and a central rifted margin segment associated with volcanism the continent ocean boundary is confined within a narrow zone 10 20 km at the sheared margin segments but is more obscure and partly masked by volcanics at the rifted margin the crustal structure and tectonic history differ significantly between the three margin segments in the north across the hornsund fault zone the crustal thickness changes abruptly from more than 30 km thick continental crusy on the svalbard platform to 6 8 km thick oceanic crust in the greenland sea this margin segment experienced oblique continent continent and partly continent ocean shear with both transtensional and transpressional component during eocene time while post eocene extension caused crustal stretching west of svalbard the volcanic province at the central rifted margin segment probably comprises an outer part of thick oceanic crust and an inner part of thinned continental crust covered by a mixture of volcanics and sediments this formed at a releasing bend in the margin in response to eocene oligocene extension and two phases of volcanism in the south the crustal thinning occurs over a wider area landward of the senja fracture zone here the margin bounds a basinal province in which as much as 18 20 km of sediments cover a highly attenuated crystalline crust the senja fracture zone evolved due to eocene transtensional continent continent shear followed by continent ocean shear and has been passive since earliest oligocone time we conclude that the first order crustal structure along the margin and its tectonic development is mainly the result of three controlling parameters 1 1 the pre breakup structure 2 2 the geometry of the plate boundary at opening and 3 3 the direction of relative plate motion the interplay between the direction of plate motion and the geometry of the plate boundary gave rise to striking differences in the structural development of the different margin segments

the evolution of the layered lower crust and the moho through geological time in western europe contribution of deep seismic reflection profiles deep seismic reflection images from a set of profiles shot in western europe have been reviewed and compared and tentative conclusions have been proposed concerning the evolution of the layered lower crust and the moho the disappearance of variscan mountain roots is related to the set up of a new moho at a typical 30 km depth and the creation of seismic layering in the lower crust deep seismic profiles suggest that these processes resulted at least in part from magmatic intrusion partial crustal melting and metamorphism of deep crustal rocks into eclogite on the other hand the layered lower crust is greatly attenuated beneath cretaceous basins and tertiary rifts in relation to prominent moho upwellings the unusual amplitude of the moho reflection and the presence of anomalously high seismic velocities in the lowermost crust beneath the tertiary rifts suggest that the moho and part of the layering are comparatively young features related to interactions between crust and mantle beneath triassic jurassic basins the layered lower crust was not affected by the subsidence of the basement with the whole crustal thinning being entirely concentrated in the upper crust this indicates that the layered lower crust and the moho were formed or restored during or after the main rifting phase seismic data reveal constraints on the processes that affect the crust mantle transition and seem to restore the moho to its typical depth after any mechanical deformation of the lithosphere

orthogonal joint systems in the bergen area southwest norway and their regional significance abstract joint orientation measurements are reported from the bergen district southwest norway the prevailing joint systems form two near vertical near orthogonal sets striking preferentially nw se and ne sw the orientation of these fissure systems is consistent with fissure systems dissecting the surface formations precambrian to quaternary throughout britain and northwest france they also show general agreement with lineaments in southern norway as revealed by landsat imagery it is suggested that the orthogonal joint network represents a fundamental tectonic structure originally implanted into the mobile outer layer of the earth in precambrian time the network has been repeatedly reactivated throughout the earth s dynamic tectonic history it was probably formed in a shear regime caused by torsional deformation associated with rotation of large scale lithospheric units which is still operating today

fennoscandian dynamics present day earthquake activity abstract the fennoscandian earthquake catalogue is constantly updated and revised and we present the best available map of earthquake activity it exhibits significant differences between areas of high and low seismicity the focal mechanisms have been determined for many of the earthquakes the total set of these focal mechanisms shows that the stress pattern is very consistent the horizontal deviatoric stresses are characterized by a grouping in the nw se directions for the maximum compression this stress field is not distorted even by the significant crustal inhomogeneity in the fennoscandian border zone this orientation of the maximum horizontal compression is precisely that expected from ridge push forces from the north atlantic ridge it is not consistent with the stress field expected from postglacial uplift after the last ice age however recently large neotectonic faults have been identified and studied in northern scandinavia and these are taken as indicators of large scale displacements shortly after the end of the last ice age seen in parallel with similar studies in scotland this shows that the stress field has changed drastically with time the present day uniform stress field and the relatively scattered distribution of observed earthquakes lead to the conclusion that the earthquakes concentrate in specific zones of brittle weak crust we claim that arguments based on tectonics must be dependent on the specific tectonic characteristics of each area separately and this remains the outstanding problem of fennoscandian earthquake activity as observed presently

seismicity and seismotectonics of norway and nearby continental shelf areas the earthquake activity of norway and nearby offshore areas is low to intermediate with few events above magnitude 5 recent significant improvements in instrumental coverage in parallel with a better utilization of older including historical data have shown that the seismicity in the south is predominantly confined to the coastal areas and to the viking graben while from the northern north sea to svalbard the earthquakes in a broad sense follow the continental margin fifty one focal mechanism solutions from these areas about half of them new reveal stress directions that clearly indicate a connection to the plate tectonic ridge push force at least for the areas at a minimum distance from the continental margin along the margin stress directions also indicate a possible connection to postglacial uplift as well as to lithospheric loading effects a dominance of normal faulting on the landward side and reverse faulting on the oceanic side agrees with this interpretation on a regional level the seismicity in these areas correlate quite well with geologic features such as grabens fault zones fault complexes fracture zones and the margin itself indicating that these structures act in a general sense as weakness zones in the presence of a regionally more stable stress field in the northern north sea however an area with quite anomalous stress orientations with strike slip faulting is found in a region transitional between normal and reverse faulting most of the earthquake foci are confined to the presumably brittle parts of the crust but many events are also located quite close to and on both sides of the moho discontinuity

deformation of the baltic continental crust during caledonide intracontinental subduction views from seismic reflection data deep seismic profiling in norway and sweden provides documentation of the amount and style of deformation of the baltic crust in response to caledonian intracontinental subduction the seismic data demonstrate that the preserved caledonide allochthon in central scandinavia is 15 to 20 km thick considerably greater than the 2 to 5 km predicted from surface geologic data much of the thickness of allochthonous rocks results from imbrication of the baltic basement within a thin skinned style thrust complex seismic data across the skardora antiform one of the basement windows in the central caledonides indicate that the window represents an antiformal stack of thrust sheets involving basement and cover rocks these data suggest that many of the basement windows in the scandinavian caledonides may result from similar thrust culminations and that the basement windows do not expose autochthonous baltic basement documentation of the response of the baltic crust during intracontinental subduction provides new constraints on reconstructions of the scandinavian caledonides and insights into the style of deformation occurring in other collisional orogens

the moho depth distribution in fennoscandia and the regional tectonic evolution from archean to permian times abstract from a variety of seismic refraction and reflection profiling results a new detailed moho depth map for the fennoscandian part of the baltic shield has been compiled a complementary crystalline crustal thickness map was compiled for denmark the skagerrak sea offshore western norway and adjacent areas where sedimentary strata reach maximum thicknesses of 10 km and even more these maps are discussed in relation to tectonic evolution and crustal ages of fennoscandia seismic velocities bouguer gravity and seismicity areas deviating from the proposed age thickness relationships are the kola peninsula archean and the oslo rift permian the average crustal thickness in the former area is around 44 km while the latter area is associated with a moho elevation of 3 5 km maximum crustal thicknesses in excess of 50 km have been found in southwestern and central finland and correlate with a crust of proterozoic age the caledonides of western norway do not exhibit as elsewhere a crustal root sub moho p n and s n seismic velocities are relatively small in that area and bouguer gravity values are strongly negative intracrustal p velocity variations do not seem to reflect geological age or specific tectonic features although the seismicity is moderate in fennoscandia most of the earthquake activity and in particular the largest earthquakes are confined to areas where the crustal thinning is most pronounced

observations of the electrical asthenosphere beneath scandinavia the recent recognition that long period i e of the order of hours electromagnetic induction studies could play a major role in the detection of the asthenosphere has led to much interest amongst the geophysical and geological communities of the geomagnetic response functions derived for differing tectonic environments experiments carried out on the ocean bottom have met with considerable success in delineating the electrical asthenosphere i e a local maximum in electrical conductivity minimum in electrical resistivity in the upper mantle in this paper observations of the time varying magnetic field recorded in three regions of scandinavia northern sweden kiruna kir northern finland northeastern norway kevo kev and southern finland sauvamaki sau are analysed in order to obtain estimates of the inductive response function c for each region the estimated response functions are compared with one from the centre of the east european platform eep and it is shown that the induced eddy currents at periods of the order of 103 104 s in the three regions flow much closer to the surface than under the platform centre specifically at a period of 3000 s these currents are flowing at depths of the order of kev 120 km kir 180 km sau 210 km eep 280 km implying that the transition to a conducting zone of 0 2 s m occurs at around these depths one dimensional inversion of ckev and ckir shows that there must exist a good conducting zone of 0 1 1 0 s m under each of the two regions of 40 km minimum thickness at depths of kev 105 115 km kir 160 185 km this is to be contrasted with eep where the d profile displays a monotonically decreasing resistivity with depth reaching 0 1 s m at 300 km finally a possible temperature range for the asthenosphere consistent with the deduced conducvitity is discussed it is shown that at present there is insufficient knowledge of the conditions water content melt fraction etc likely to prevail in the asthenosphere to narrow down the probable range of 900 1500 c

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

climate variability during mis 20 18 as recorded by alkenone sst and calcareous plankton in the ionian basin central mediterranean abstract this study shows the first mediterranean high resolution record of alkenone derived sea surface temperature sst in the marine sediments outcropping at the ideale section is southern italy central mediterranean from late marine isotope stage mis 20 through early mis 18 the sst pattern evidences glacial interglacial up to submillennial scale temperature variation with lower values 13 c in late mis 20 and substage 19b and higher values up to 21 c in mis 19c and in the interstadials of mis 19a the sst data are combined with the new calcareous plankton analysis and the available chronologically well constrained carbon and oxygen isotope records in the is the multi proxy approach together with the location of the is near the italian coasts the lower circalittoral upper bathyal depositional setting and high sedimentation rate allow to document long and short term paleoenvironmental modifications sea level rainfall inorganic organic fresh water input to the basin as a response to regional and global climate changes the combined proxies reveal the occurrence of a terminal stadial event in late mis 20 here med htix and warm cold episodes here med batix and med ydtix during termination ix tix which recall those that occurred through the last termination ti during these periods and the following ghost sapropel layer insolation cycle 74 784 ka in the early mis 19 high frequency internal changes are synchronously recorded by all proxies the substage mis 19c is warm but quite unstable with several episodes of paleoenvironmental changes associated with fluctuating tropical subtropical water inflow through the gibraltar strait variations of the cyclonic regime in the ionian basin and the southward shift of westerly winds and winter precipitation over southern europe and mediterranean basin three high amplitude millennial scale oscillations in the patterns of sst and calcareous plankton key taxa during mis 19a are interpreted as linked to changes in temperature as well as in salinity due to periodical water column stratification and mixing the main processes involved in the climate variability include changes in oceanographic exchanges through the gibraltar strait during modulations of atlantic meridional overturning circulation and or variations in atmospheric dynamics related to the influence of westerly and polar winds acting in the paleo ionian basin a strong climate teleconnection between the north atlantic and mediterranean is discussed and a prominent role of atmospheric processes in the central mediterranean is evidenced by comparing data sets at the is with italian and extra mediterranean marine and terrestrial records

deep seismic survey images crustal structure of tornquist zone beneath southern baltic sea the tornquist zone is europe s longest tectonic lineament and bisects the continent in a nw se direction from the north sea off nw denmark to the black sea new deep seismic reflection and coinci

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

metallogeny and lead isotope data from the oslo paleorift abstract the permian oslo paleorift situated at the southwestern margin of the baltic shield includes the oslo region and its southward extension under skagerak the exposed part of the paleorift comprises two half grabens vestfold and akershus graben segments filled with permian sediments volcanics and intrusive rocks as well as cambrc silurian sedimentary rocks the evolution of the oslo rift can be divided into a rifting stage a caldera stage and a batholith stage the rifting stage is characterized by weak mineralization of native copper in basalts the caldera stage includes fe ti cumulates in subvolcanic layered alkali gabbros nb ree disseminations in aphyric trachytes and epigenetic mo mineralizations the batholith stage includes porphyry and intraplutonic vein deposits of mo and contact metasomatic deposits of fe w mo be zn pb cu bi as and f along the rift margin in precambrian gneisses and amphibolites there are vein deposits of fe oxides base metals and native silver the relationship between the rift margin deposits and the evolution of the rift is still unclear but many deposits are spatially related to permian dolerite dykes lead isotope data from hydrothermal deposits in the oslo rift fall into two main groups the first group comprises the rift margin deposits they are very radiogenic and reflect an upper crustal source for the metals the source of the silver deposits generally has a lower value and a higher w value than the base metal deposits the rift margin deposits are therefore not a product of metal zonation from a single hydrothermal solution the second group comprises deposits associated with granitic intrusions in the oslo graben and reflects a lower to intermediate crustal source for the metals deposits in the vestfold graben segment are generally more radiogenic than deposits in the akershus graben segment and this difference may be explained by a small contribution of highly radiogenic lead from underlying precambrian granites the data show that individual intrusions can be characterized by their lead isotope composition and thus confirm the relationship between intrusions and mineralizations the data indicate a more radiogenic source for the peralkaline intrusions compared to the biotite granites but more data are needed to confirm this suggestion

simple shear deformation of the skagerrak lithosphere during the formation of the oslo rift abstract simple shear deformation of the entire lithosphere has been postulated by wernicke 1985 and others but up to now unequivocal seismic evidence in support of this hypotheses has been lacking here we describe well defined seismic reflectors below the skagerrak sea one of which is interpreted as a low angle fault underlying the skagerrak graben segment of the permian oslo rift this reflective lineament can be traced from the mid crust through the lower crust offsetting moho and continuing downwards to ca 50 km depth 16 s a separate mantle reflection beneath the graben may be associated with an earlier period of thrusting the 1730 km of deep seismic reflection data in skagerrak indicate that the crust and mantle inherited a pronounced structural fabric from the proterozoic grenvillian sveconorwegian orogeny during formation of the oslo rift reactivation of these implied weak zones as localized detachment planes would explain the extensional deformation style of the non magmatic skagerrak graben

lithospheric thermal structure in the baltic shield summary steady state heat conduction modelling was carried out to calculate the crustal temperature field and thermal lithosphere thickness in the baltic shield the radiogenic sources at the surface were fixed depending on the age of the crust below the uppermost 10 kilometres a relationship between heat generation and p wave velocity was applied and seismic structure was used to define individual crustal blocks of specific thermal parameters small scale surface heat flow density anomalies are interpreted as lateral variations of heat generation within the upper part of the crust whereas the large anomaly in the southern part of the shield is attributed to an anomalously high mantle heat flow density the results are shown on maps outlining mantle heat flow density moho temperature and thermal lithosphere thickness a relationship between subcrustal temperature and moho depth was found except for southern sweden the lithospheric thickness is found to exceed 200 km in the bothnian gulf northern central finland and part of the kola peninsula where temperatures at the moho are less than 500 c the temperature gradient at the top of the mantle has on the average a value of 7 5 mk m and the mantle heat flow density varies from 19 to 25mwm 2 towards the south the lithospheric thickness decreases until it attains a value lower than 100km under southern sweden in this latter area the moho temperature and mantle heat flow density are remarkably high 700 900 c and 30 45 mw m 2 respectively as well as the temperature gradient which amounts to 10 mk m

event location at any distance using seismic data from a single three component station this paper demonstrates the usefulness of a novel technique for analysis of single site three component seismogram records for reliable epicenter determinations for even small seismic events at any distance parameters that may be extracted by this technique are p phase identifications in terms of x 2 probabilities and the associated slowness vectors using data from short period and broadband seismometers installed in the new noress norway array single site event locations are demonstrated azimuth estimates are found directly from the slowness vector while epicenter distance estimates are derived from differential arrival times between pn pg sn and lg sg simultaneous inversion for crustal model and hypocenter data indicate a crustal thickness beneath noress of 33 5 km if sn and or lg are well recorded focal depth estimates are often accurate to within a few kilometers locating teleseisms is problematic since secondary phases are not fully readily identifiable and hence distance can be estimated only by conversion of the measured apparent phase velocity for distances out to approximately 1000 km event location errors as compared to network solutions seldom exceed 50 km and are due mainly to errors in azimuth estimates with sn phases available focal depth errors appear to be of the order of 4 km for teleseismic p waves short period location errors occasionally exceeded 7 and were due mainly to poor distance estimates using broadband records from relatively strong events location errors were about 1 further refinements seem feasible by using seismicity information and or recognition of stationarity in the p wave decomposition pattern for events within the same region at least in certain cases such approaches reflect genuine location refinements as demonstrated

crust and upper mantle structure along the dss baltic profile in se finland summary the 430 km long dss baltic profile in se finland runs through the rapakivi intrusion the svecokarelian geosynclinal complex including the ladoga bothnian bay zone and the archean basement complex a brief description of field operations in 1982 is presented record sections of p and s waves were used for the interpretations a starting model was constructed by inverse methods and improved by dynamic ray tracing modelling three blocks are indentified on the cross section from sw to ne these are the southern associated with the rapakivi massif depth to moho m about 40 km the central ladoga bothnian bay zone with m of approximately 55 60 km and the northern archean crystalline basement with m about 40 45 km the average crustal velocity is 6 6 6 7 kms the crustal structure along the baltic profile is compared with the surrounding dss profiles finnish sveka and soviet kern tulos and ladoga profiles the eastern border of the ladoga bothnian bay zone was located and specific features of the crust and uppermost mantle along the profile area are discussed

evidence for early proterozoic plate tectonics from seismic reflection profiles in the baltic shield plate tectonics provides the linking framework for all tectonic and magmatic activity seen today but it is not known when plate tectonics first developed on earth new deep seismic reflection and

geoelectric studies and conductivity structures of the eastern and northern parts of the baltic shield abstract large resistive regions ideally having a horizontal size of about 200 km are needed to study reliably the conductivity distribution of the lower crust of the baltic scandinavian shield by deep electromagnetic em methods the overburden of the shield consists of quaternary sediments and is thin with a low conductance this makes the shield favourable for em studies the numerous conductive zones e g schist belts in the uppermost crust however require careful two dimensional modelling in order to obtain reliable electrical cross sections of the earth s crust electromagnetic methods magnetometer array studies and magnetotelluric profiling have proven to be a useful tool in studies of the lithospheric geophysics of the shield the crust of the finnish and soviet baltic shield can be divided into geoelectrically different blocks in central finland the blocks are surrounded by narrow highly conductive band like formations they are located in the middle crust and their upper surface often occurs in the 6 10 km region the resisitivities are typically 5 10 m the greatest anomalous crustal regions have a conductance of more than 20 000 s the lower crust is very conductive in most parts of the shield two dimensional models of long magnetotelluric profiles indicate the great variety and complexity of the geoelectric structure of the crust

seismic modelling of the norwegian danish basin along a refraction profile in northern jutland abstract the seismic velocity of the sedimentary structure of the norwegian danish basin and its underlying precambrian basement have been modelled by 2d ray tracing interpretation of a 200 km section of a deep refraction seismic profile in northern jutland eugeno s project profile 3 a model is presented which satisfies the quite detailed prior geological information from wells and shallow reflection seismic interpretations for the upper sedimentary layers and the traveltime observations from three shotpoints along the profile the model shows a basin thickness of about 7 km in the southern part of the profile thinning to 1 5 2 km in the northern part towards the fennoscandian border zone in the central part the model thickness is about 10 km sedimentary p wave velocities vary from 1 9 to 3 8 km s in the upper layers quaternary tertiary and upper cretaceous units to about 5 5 km s in the deepest palaeozoic unit the precambrian basement shows velocities of 5 9 6 5 km s there is a clear indication of a basement reflector at depths of 11 km velocity contrast 6 0 6 4 km

magnetotellurics along the fennoscandian long range profile summary estimates of magnetotelluric transfer functions along the fennoscandian long range fennolora profile are presented and discussed in relation to the major lithotectonic subdivisions of the swedish part of the baltic shield none of the transfer functions are found to be consistent with a one dimensional earth structure thereby making a quantitative interpretation difficult however the geological units are found to display characteristic differences in the obtained transfer functions and in some areas a quantitative interpretation can be justified the most interesting structure indicated is a low resistive 4 am crustal structure of lateral extension more than 150 km centred around the skellefte ore district a minimum thickness for this structure of 15 km is inferred

deep crustal structure of the northern north sea viking graben results from deep reflection seismic and gravity data abstract the combination of deep reflection seismic interpretation nsdp 84 data set and 3 d modelling of free air gravity data resulted in a regional crustal model for the viking graben area the interpretation focuses on the structure of the crystalline crust which is displayed by depth sections a map of the moho a map of the thickness of the crystalline crust and a map of the extension factor the thickness of the crystalline crust varies from 10 km near the centre of the graben to 30 km under the adjacent platforms the thickness of the sedimentary cover varies from 11 km to 1 km respectively the topography of the moho reflects in a highly smoothed way the top of the crystalline basement the seismic moho coincides with the gravity moho although some discrepancies are observed in the most extensively stretched areas high values are observed but no indications of the presence of oceanic type crust associated with a crustal rupture have been found the results are thought to favour a stretching model of graben development a strong inclined reflection and associated reflection patterns in the east shetland platform are tentatively interpreted as a fragment of the iapetus suture the asymmetrical structure of the graben is assumed to be associated with initial inhomogeneities in the crust

an introduction to the european geotraverse project first results and present plans abstract the european geotraverse egt is an international multidisciplinary project focused on a north south orientated lithospheric profile 4000 km long and of varying width extending from northernmost scandinavia to north africa this profile consists of three interlinking segments northern central and southern comprising a continuous succession of tectonic provinces ranging from the oldest precambrian areas of the baltic shield to the currently active area of the western mediterranean the broad aim of the egt project is to obtain a better three dimensional picture of the structure state and composition of the continental lithosphere to use as a basis for an understanding of its evolution and dynamics all of the 12 major projects that constitute the egt joint programme have now been initiated and several of these projects are nearing completion along the northern segment data from the fennoscandian long range project fennolora a 2000 km long seismic profile across the precambrian baltic shield show that except beneath southern sweden the shield is characterized by a high velocity 40 50 km thick crust including a 5 10 km thick crust mantle transition zone an alternating series of 4 6 high and low velocity zones is present in the subcrustal lithosphere the base of which increases in depth from ca 110 km to ca 230 km from south to north beneath the shield the top of the mantle transition zone lies at a depth of about 450 km the second major project along this segment the eugeno s european geotraverse northern segment southern part project is a multidisciplinary study of the fennoscandian border zone and was largely completed in 1984 with the realization of a large scale seismic experiment preliminary interpretation of the excellent data obtained indicate the presence of strong lateral variations in internal crustal structure beneath the danish basin field work for a third major project a multidisciplinary transect of the archaean and early proterozoic terrains in the northernmost part of the shield the polar profile was carried out in 1985 a series of deep seismic reflection lines has so far been realized in the area of the central segment in the context of german national programmes first interpretations of the seismic data from a 260 km long profile across the two main intra variscan hercynian lineaments have shown the presence of numerous horizons making up a highly reflective zone in the lowermost 10 km of the crustal section studied and distinct changes in reflectivity between the main variscan tectonic zones in 1986 the entire segment will be investigated in detail in an ambitious international programme of integrated geological and geophysical studies a series of seismic experiments termed egt s have been carried out across the southern segment in 1982 1983 and 1985 interpretation of data from the 1982 and 1983 experiments have led to several interesting results including 1 1 the suggestion that two crust mantle like interfaces exist beneath the po basin at depths of about 35 and 50 km and adjacent tectonic units these interfaces marking a deep contact zone between the adriatic and european plates 2 2 in the area between genoa and corsica the ligurian sea is underlain by a greatly thinned distinctly layered section of continental crust and 3 3 corsica and sardinia are underlain by bowl shaped typically variscan continental crusts the 1985 phase of seismic surveys focused on crustal structure beneath tunisia and the adjacent seas in addition two off traverse projects are being realized first a wide aperture network of autonomously recording seismic stations nars installed along the line gothenburg malaga between 1982 and 1984 is already yielding high quality data on the upper 600 700 km of the mantle second an investigation of lithospheric seismic anisotropy in the area of the iberian peninsula is being organized for 1987 1988 finally of great importance are the systematic compilation of existing data and where needed in critical regions collection of new geophysical and geological data presently being carried out for the entire area encompassed by the egt it is expected that these compilations will be completed by 1987 at about the same time that full results from the main large scale seismic experiments become available enabling the construction of an integrated lithospheric cross section along the egt requiring a final phase of intensive multidisciplinary collaboration

lithospheric structure in southern sweden results from fennolora abstract the major objective of fennolora the fennoscandian long range seismic project of 1979 was the determination of lower lithospheric and upper mantle structure down to depths in excess of 400 km below the fennoscandian shield in the component represented by this study data recorded at stations in southern sweden from three shotpoints one in northern germany profile wn and two separated by 300 km in southern sweden profiles bn and cs are used to derive a two dimensional lithosphere structure model extending over 600 km from north to south the profile crosses the svecofennides the smaland varmland granite belt paleozoic cover rocks lying unconformably on the fennoscandian shield the baltic sea where there were no stations and into the caledonian tectonic province of northern germany where one shotpoint but no stations were located interpretation of the three record sections was based on a two dimensional ray tracing procedure which included the calculation of asymptotic ray theory synthetic seismograms lack of data from 0 150 km for the shotpoint in germany to the first station in southern sweden precluded derivation of any detailed crustal structure for this region crustal thickness was determined to be 32 km a rapid increase in velocity from 8 0 to 8 35 km s at about a depth of 50 km is underlain by a low velocity zone extending to a depth of 70 km the reversed profiles bn and cs in southern sweden are fundamentally different interpretation shows a difference in crustal thickness of about 12 km with the northern segment having a deeper moho a feature resulting from a thicker lower crust and a crust mantle transition zone rather than a rapid increase in velocity in the southern segment of the reversed profile a crustal low velocity zone of limited extent is indicated the most significant aspect of the interpretation is the suggestion that the change in crustal structure between the two shotpoints in southern sweden occurs within a transition zone of limited lateral extent less than a few tens of kilometers we suggest that the model transition zone is a fundamental lithosphere boundary associated with the juxtaposition of the smaland varmland granite belt and the svecofennides

deep temperatures and lithospheric thickness along the european geotraverse abstract one dimensional steady state heat conduction models in the crust and upper mantle for 24 areas along the european geotraverse egt show that temperatures are higher beneath young geological regions at the moho they range from 450 to 825 c heat flowing out from the mantle varies from 18 to 72 mw m2 and the contribution of crustal heat flow density evaluated by the correlation between seismic velocity and the abundance of radioactive elements accounts for about 48 of the surface heat flow density the temperature gradient below moho from about 5 to 23 mk m is low where the surface heat flow density reaches its minimum values the depth at which the geotherms reach 0 85 of the mantle melting temperature is greater in the precambrian and caledonian areas with a maximum value of about 195 km under the early proterozoic scandinavian shield in the hercynian and alpine areas this depth is generally lower higher values maximum 150 km coincide with the orogenic belts and the continental block of sardinia and corsica under the ligurian sea and the sardinian channel the lithospheric thickness decreases remarkably to about 50 km globally the lithosphere thickness is controlled by the mantle heat flow density which decreases according to a power law with an increase in the lithosphere asthenosphere depth

deep crustal earthquakes in the southern baltic shield summary on 1986 july 14 one of the largest earthquakes in the baltic shield during this century occurred near skovde in the province of vastergotland sweden with a magnitude of ml upp 4 5 it was followed by the so far largest number of recorded aftershocks more than 20 from any swedish earthquake the strongest aftershock with ml upp 3 4 occurred about one hour after the main shock a few months later on november 2 an ml upp 3 6 event took place near mariestad some 30 km northwest of the skovde series all these shocks were located in the lower crust with foci at depths between 20 and 35 km indicating active movements in the shield at depths where mainly ductile deformation is usually assumed waveform modelling seems to be an excellent tool to obtain information about the focal depth which is otherwise hard to determine the calculated seismic moment and stress drop for the skovde main shock are 5 9 1014 n m and 2 8 mpa respectively and for the mariestad earthquake 2 3 1014 n m and 7 3 mpa respectively focal mechanisms for the skovde main shock and largest aftershock have been obtained from p wave polarities and synthetic modelling and for the mariestad earthquake from p wave polarities the synthetics show good agreement with short period seismograph records for frequencies up to at least 2 hz and distances up to at least 200 km a comparison of the mechanisms indicates that the faulting of the area is complex with styles ranging from strike slip to normal suggesting that other factors than the push from the north atlantic ridge e g post glacial rebound also contribute to the lithospheric stress pattern this explanation is corroborated with findings from nearby southern norway

the oslo graben gravity high and taphrogenesis abstract the paleo oslo rift oslo graben system is geophysically characterized by a pronounced gravity high a reinterpretation of gravity data in the region using a flexible inversion scheme indicates that the causative body has a density contrast of 0 06 g cm 3 the inversion results also suggest that this body is most likely located in the upper part of the lower crust and extends eastward well outside the graben area proper estimates of crustal thicknesses before and after the rifting aborted suggest that rifting commenced in the south and then progressed northwards the corresponding pole of rotation was located approximately 240 km nne of the city of oslo the hypocentres of 7 small local earthquakes were found to be located on the periphery of the anomalous body as derived from gravity modeling seismic investigations including 3 d imaging using norsar data have failed to provide conclusive evidence on intrusion of asthenospheric material in the lithosphere as part of the oslo graben taphrogenesis the explanation proposed here is that the geophysical imprints of oslo rift oslo graben evolutionary processes within the upper mantle have been considerably weakened over the associated time span of some 150 200 ma finally it is hypothesized that the asthenospheric injection into the lithosphere took place over a relatively small cross sectional area of the moho the further ascent of the gabbroic magma was partly hindered by the relatively rigid upper crust thus causing extensive lateral flow eastwards outside the oslo graben proper in the 10 20 km depth range

uncharacteristic earthquakes on the san andreas fault one model of earthquake recurrence holds that large quakes exhibit regular or characteristic rupture patterns combined with a simple picture of how elastic strain accumulates in the faults this implies that characteristic earthquakes occur at regular intervals in her perspective grant proposes that recent data from study sites along the san andreas fault are difficult to understand with this model the earthquake activity is more irregular and more clustered than the model would suggest

observations of seismic reflectors in the lower lithosphere beneath the skagerrak the lithosphere the rigid outer shell of the earth includes both the crust and the upper mantle in the past decade the complex continental crust and underlying shallower mantle lithosphere have been extensively studied using reflection seismology but the lower lithosphere has not been successfully imaged here we report the observation of continuous and virtually horizontal reflectors in the lower continental lithosphere down to depths of 100 110 km below the skagerrak scandinavia these are apparently not structurally associated with the crust whereas previously reported mantle reflectors such as those observed by the birps group around the british isles1 2 are often located near large faults in the overlying crust like the crust the mantle lithosphere shows a seismically transparent upper and a reflective lower part thus the processes that form seismic reflectors seem to have an affinity for ductile rheological environments in the mantle as well as in the crust our observations are consistent with a model consisting of a mechanically strong upper lithosphere underlain by a thermal boundary layer which separates it from the convecting asthenosphere

the crustal structure of western europe a thorough understanding of the tectonic processes that formed the con tinental lithosphere under changing geometric and physical conditions through time requires detailed knowledge of the structure of the crust one of the best locations to study this fundamental problem is the northern and western part of europe because it is made up of a number of tectonic provinces ranging in succession from the oldest precambrian areas of fennoscandia to the currently active regions of the mediterranean sea the geology of europe a region extending from the northern tip of scandinavia north cape southward to north africa encompasses the pre 2500 ma archean craton in the northernmost part of the baltic or fennoscandian shield the proterozoic paleozoic and cenozoic pro vinces of northern and central europe that have been added on to this nucleus and the active transition zone between the eurasian and african plates in the mediterranean area cf figure 1 from a tectonic geologic point of view europe can be divided into four main units that cover the whole time span of the history of the continental crust

rupture creep lamellae and crocodiles happenings in the continental crust from the observation of certain seismic reflectivity patterns in phanerozoic continental crusts a relationship is suggested between 1 rupture and seismicity generally restricted to the upper crust 2 crocodile tectonics a kind of vertical indentation observed by diverging plane reflectors preferably in the middle crust 3 creep and the formation of reflecting lamellae in the lower crust based on these observations and on viscosity depth estimates a conceptual model for the above mentioned structures is presented which is also compatible with observations in older crusts boundary conditions are given for rupture as well as for lamellae forming creep

crustal structure of the fennoscandian shield a traveltime interpretation of the long range fennolora seismic refraction profile abstract the 1979 fennoscandian long range project fennolora was aimed at the determination of the detailed structure in the earth s mantle down to a depth of about 400 km observation distances reached almost 2000 km within scandinavia between shotpoints off the north cape and the southern coast of sweden to achieve an unbiased regarding the upper mantle structure a careful crustal survey was carried out along the entire profile at the same time beneath the fennoscandian shield i e the central section of the profile the crust is characterized by quite a smooth increase in p wave velocity down to the moho which lies at a depth of about 50 km in the southern half of the shield and at about 45 km further north the mean crustal velocity is 6 6 6 7 km s at the base of the crust the velocity increases gradually from about 7 km s to 8 0 8 4 km s in a 5 10 km thick crust mantle transition zone both in the south and in the north the relatively homogeneous crust of the baltic shield borders on areas with a more differentiated velocity structure first order discontinuities at shallower depth characterize the crust mantle boundary near the southeastern tip of sweden 38 km and under the caledonides in the north 43 km

explosion generated short period surface wave dispersion and noise studies along linear seismic arrays in southern sweden summary the dispersion of short period fundamental mode rayleigh waves rg was measured along linear arrays on the baltic shield the data used is a selected set of 176 records along three refraction profiles in southern sweden the phase velocity determined in the 0 2 3 5 hz band could be grouped into a number of dispersion regions each regional dispersion showed a high internal consistency and the slowness as function of frequency was almost linear in the 1 3 hz band linear regression was used to separate the effects of the medium from the experimental uncertainties systematic studies of the noise to signal ratio showed that the standard deviation of the dispersion within each region and the above frequency band essentially reflected the lateral heterogeneities from the dispersion shear velocity models were inverted down to about 2 3 km in one case down to 6 km the s wave velocity was weakly constrained to a non decreasing function of depth with decreasing gradient the p wave velocity was constrained using recent results on poisson s constant ranging from 0 28 to 0 25 in the 0 2 km interval the density was constrained using geological knowledge of the area using these constraints the s wave velocity increased rapidly with depth in the first few hundred metres of the crust the models could be naturally grouped into three sets with similar character in close agreement with the large scale surface geology the highest velocities were found in the east in the smiland varmland granitic belt and the smallest in the northwest in the sveconorwegian crust

crustal structure along the northern fennolora profile abstract many data notably concerning the great change in earth s bulk heat flow which has occurred since the archaean era suggest that late archaean and early proterozoic tectonic processes were most probably different from phanerozoic plate tectonics preserved structures in the earth s crust revealed by the seismic refraction wide angle reflection method can help in understanding the different crustal tectonic processes that have influenced the evolution of the area p wave analysis for the northern part of the fennolora deep seismic refraction line clearly shows the difference between the central swedish svecofennian crust and the archaean crust in the north along the archaean part of the fennolora line high apparent velocities have been recorded in the upper crust the central svecofennian crust on the other hand shows normal upper crustal velocities data from the archaean part of the fennolora profile give support to the concept of an ensialic development in this region during the late archaean and early proterozoic

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

crustal evolution of fennoscandia palaeomagnetic constraints abstract palaeomagnetic poles from fennoscandia ranging in age from archaean to tertiary are compiled and graded using a modified briden duff classification scale an new filtering technique is applied to select only the most reliable poles for analysis the filtering takes into account the following information 1 1 source block of rock unit 2 2 age of rock 3 3 age of magnetization component 4 4 scatter of palaeomagnetic directions 5 5 information from multicomponent analysis of natural remanent magnetization nrm 6 6 whether the pole considered belongs to a cluster or subcluster of poles 7 7 magnetic polarity and 8 8 the author s original assignment of results data are still insufficient for the drawing of separate apparent polar wander paths apwp for different blocks or cratons of fennoscandia treating fennoscandia as a single plate a new apwp from archaean to permian is constructed from the five previously drawn apwp loops or hairpins only one the jatulian loop 2200 2000 ma disappears in filtering the loops during 1925 1700 ma and during 1100 800 ma ago are linked to svecofennian and sveconorwegian orogenies respectively palaeomagnetic data support the concept that these orogenies took place episodically three distinct orogenic pulses early middle and late can be distinguished in the cluster plots of palaeopoles the drift history of fennoscandia from archaean to permian is presented during most of geological history fennoscandia has occupied low to moderate latitudes and undergone considerable latitudinal shifts and rotations the svecofennian and sveconorwegian orogenies have different kinematic characteristics during the svecofennian orogeny fennoscandia drifted slowly while rotating a large amount in an anticlockwise sense during the sveconorwegian orogeny it drifted rapidly and rotated first clockwise and then anticlockwise the most striking feature in the drift velocity curves is however the pronounced maxima in the latitudinal drift and rotation rates 9 cm yr and 0 8 ma respectively during the late subjotnian jotnian anorogenic magmatism and rifting phase 1450 1250 ma ago possibly reflecting the passage of fennoscandia across a thermal upwelling hotspot at equatorial latitudes the use of palaeomagnetism in delineating and dating movements between blocks is demonstrated with three examples from the polar profile area the northernmost section of the european geotraverse

twenty years of deep seismic reflection profiling in germany a contribution to our knowledge of the nature of the lower variscan crust near vertical reflection studies in germany have concentrated on the following objectives 1 the nature of crustal reflectors 2 a detailed velocity structure 3 zones of increased tectonic activity e g faults approximately 20 years ago the identity between reflectors in the near vertical and the wideangle range was established wide angle events being strongly influenced by velocity gradients a pronounced but limited low velocity body below the urach geothermal anomaly was detected by using an especially long spread length an alternating polarity of deep crustal reflectors could be demonstrated a deep fault zone cutting the whole crust at steep angles and shallow angle overthrust structures were detected in the variscan area there is generally a strong reflectivity in the variscan lower crust between the conrad level at 6 s two way travel time i e 18 km and the moho at 9 10 s i e 27 30 km depth the lower crust seems to consist of thin lenses or lamellae with alternating velocity they were probably generated by an extensive synand post orogenic melting process leaving behind mafic and ultramafic splinters in the lower crust and a compact ultramafic residue in the uppermost mantle the moho being formed as a new and uniform boundary the poorly reflective upper crust is probably dominated by vertically oriented plutons in a rigid environment

an oslo graben experiment shooting at sea and recording on land abstract during the eugeno s field work in 1984 a large number of airgun shots was fired along a line between larvik norway and lysekil sweden the signals were recorded on land at larvik using a stationary v shaped land array with 48 recording channels the airguns were fired every 300 m along the profile with an effective recording distance range of 30 150 km but data quality deteriorated rapidly beyond 80 km a hybrid variant of slant stacking was used in record analysis with the robust semblance technique for estimating velocity spectra when the various p traveltime branches were properly located in the p plane the inversion of the corresponding kinematic parameters to a 1 d velocity depth model was minor the main results are a presumed 5 km thick upper weathered layer with a velocity of 5 6 km s 1 a 5 13 km thick intermediate layer with a velocity of 6 3 km s 1 and a 13 31 km thick lower crustal layer with a velocity of 6 8 km s 1 the weathered layer is exceptionally thick and the intermediate layer seems to typify the transition between upper and lower crust in a typical oslo graben tectonic setting

seismic imaging of upper crustal basement faults in the skagerrak sea abstract the continental upper crystalline crust appears almost non reflective on many deep seismic reflection profiling sections typically only the major faults and lithological boundaries are imaged whereas smaller scale faults are usually not identified a deep seismic survey 0 16 s twt covering the skagerrak sea was undertaken in 1987 although the northeastern part of the skagerrak has only thin or no sediment cover the data in the depth range 0 6 s twt were reprocessed to commercial exploration standards close inspection of these high quality sections provides evidence of abundant moderately steep dipping 20 50 faults that can be traced to depths of 10 13 km fault geometry which can be inferred at profile intersections is in accordance with the general tectonic fabric observed on land and beneath the skagerrak sea these areas were deformed during the proterozoic sveconorwegian orogeny and during the formation of the permian oslo rift due to post permian erosion of the skagerrak sea proper the relative age and movements of the basement faults could not be determined this study shows that under favorable conditions faults in the upper crystalline crust are mappable by seismic means such measurements have the potential for resolving variations in block geometry with depth and also recent seismotectonic movements

growth of the continental crust a planetary mantle perspective abstract this paper addresses the net growth through time of the total continental crust cc not to be confused with sporadic growth of individual continental masses e g by accretion of island arcs a process that does not necessarily require net growth geochemists tend to emphasize differences between cc and oceanic crust oc but from a whole planet perspective cc and oc are remarkably similar in composition and density the cc oc distinction is canonically defined in terms of elevation however the earth s bimodal elevation pattern clearly reflects a genetic distinction the oc participates directly in mantle convection whereas the cc is the remaining relatively stable portion of the crust the earth s advective cooling mechanism requires massive subduction of lithosphere which due to the inherent buoyancy of crust tends to require a low crust total lithosphere thickness ratio given relatively constant lithosphere thickness except near thermally buoyant mid ocean ridges the earth responds by heaping most of its crust into a few thick continents allowing subduction to freely operate over a large area of relatively thin crust this crustal thickness bimodality engenders the bimodal elevation pattern prevailing wisdom tends to hold that the rate of cc growth has been slow and ongoing typical models show cc volume roughly proportional to 1 h where h is the total heat generation by the earth s k th and u however such models are generally vague as to the mechanism that supposedly holds back growth even after 4 5 ga the strong t dependence of mantle rheology probably ensured that the mantle took only a few million years to cool within a few hundred degrees of its modern t gradual compositional evolution of the cc from a plausible initial composition e g basaltic anorthositic might have slightly lowered the mean cc density but this effect is of second order importance in comparison to the general increase in crust total lithosphere thickness ratio for any given crustal thickness engendered by temporal cooling of the mantle intense cratering occurring coevally with early intense crust mantle recycling probably reconstituted virtually all of the cc extant prior to 3 9 ga but cc was probably not recycled into the mantle any faster than it was regenerated lunar samples and meteorites provide strong circumstantial evidence that in primordial times the outer few hundred kilometers of all terrestrial planets were largely molten outcomes of primordial magmatism must have been strongly dependent on the pressure regimes within the planetary interiors in general the far greater dt dp of silicate melting curves vs adiabats probably tended to foster a layered magmasphere with a partially molten or paracumulate layer constrained by dt dp relationships to a p range of roughly 3 gpa surmounted by fully molten magma ocean layer also limited in thickness by efficient convective cooling in the earth p stabilized garnet and al rich pyroxene were key factors limiting ultimate yield of crustal feldspar this effect has been examined with quantitative models of high p fractional crystallization including in some models periodic replenishment of the magma ocean from below results indicate that a plausibly deep 300 400 km magma ocean would produce a crust comparable in both composition and thickness to that of the modern earth mildly worrisome discrepancies involve cao for which high p pyroxene compositions are poorly constrained and sio 2 the crust probably evolved to its present sio 2 rich composition through differentiation catalyzed by h 2 o which presumably was not as abundant in the magma ocean as in the crust soon afterward nominal model results for mean crustal thickness are typically 1 2 1 3 the modern global mean but for this parameter the model is designed to give conservatively high results so the discrepancy is probably not significant after the chaotic first few million years of solar system history i suggest that planetary size and composition for more than time govern crustal volume

crustal reflectivity in the skagerrak area abstract reflectors within the crystalline crust are often observed in deep seismic reflection profiling surveys the lower crust in extensional areas is generally credited with an abundance of reflectors the deep seismic reflection data 16 s twt from the m v mobil search cruise in skagerrak show a reflective lower crust and a relatively transparent upper crust in most of the area reflectivity seems to be less inside the oslo rift and also beneath the sediment covered areas reflectivity maxima are found near the moho and at depths of 10 20 km the latter is taken to coincide with the transition between the brittle upper and ductile lower crust the distribution of crustal reflectors in skagerrak and their possible relationships with seismic velocities earthquake depth distribution and major tectonic elements such as the fennoscandian border zone the oslo rift system and the shield environment are discussed hypotheses on the formation of the crustal reflectors are also briefly reviewed

crustal structure of the barents sea from seismic data abstract in 1976 the institute of physics of the earth and the institute of oceanology the u s s r academy of sciences carried out deep seismic soundings in the barents sea along a profile 700 km long northeast of murmansk a system of reversed and overlapping traveltime curves from 200 to 400 km long has been obtained the wave correlation was effected by several independent approaches which identified on the records the refracted and reflected waves from boundaries in the earth s crust and the upper mantle different methods were applied for the solution of the inverse problem the isochrone method the intercept time method and the iteration method the use of these different methods gives an indication of the general applicability of the interpretation and of the most reliable elements in the seismic model all the interpretations and representations of the section positively establish an essentially horizontal inhomogeneity of the earth s crust in the barents sea on the whole the structure is similar to that of deep sedimentary basins of the east european platform the thickness of the sedimentary layer varies from 8 to 17 km the average crustal thickness is about 35 40 km the velocities in the upper part of the consolidated crust are 5 8 6 4 km s in the lower crust they are 6 8 7 0 km s and higher

skagerrak evolution derived from tectonic subsidence abstract stratigraphic data gathered from 32 points in the continental skagerrak basin have been analyzed by basin modelling methods non tectonic effects of sediment loading were removed by backstripping giving the observed tectonic subsidence this was compared to synthetic subsidence curves constructed on the basis of a combined lithospheric extension and metamorphism model where gabbroic magma which was emplaced during the permian is transformed to granulits eclogite facies the overall picture is that prior to 230 ma subsidence was roughly similar north and south of the fennoscandian border zone fbz after this time the northern area sagged due to the still preserved lithospheric thermal anomaly and continued metamorphism whereas the southern area was also affected by extension a physical explanation for this contrast in evolution is given by considering the relative lithospheric strength history of the areas the northern area becoming progressively stronger with time relative to the southern area as crustal thicknesses are reduced total post 570 ma extension is approximately 1 4 40 north of the fbz and 1 5 50 south of it north of the fbz the average thickness of the metamorphic layer is between 3 and 7 km whereas south of the zone a thickness of 1 2 km is estimated this difference may reflect variations in the degree of permian magmatism the temperature history of the sediments and the degree of organic maturation has been calculated by the tti method the youngest sediments to have experienced temperatures high enough for significant oil generation are estimated to be of bathonian age the present model heat flow of about 57 mw m 2 is in agreement with observations lateral displacements along the fbz are of the order of 10 20 km and have consequently only played a minor role in the development of the skagerrak basin a thermal lithosphere thickness of about 125 km is inferred

reflection seismic investigations of the siljan impact structure sweden reflection seismic measurements were carried out during 1985 in the area of the siljan impact structure of sweden a total profile length of 80 km was shot to get a clearer picture of the subsurface geometry of impact structures and the processes that formed them than could be obtained from surface observations or theoretical modeling studies standard stacked sections have been interpreted in terms of current impact models the transient crater diameter is found to be approximately 22 km considerably smaller than current estimates based on interpretation of surface geology the structural uplift is estimated to be 3 km at the 5 to 8 km level and the material flow necessary to support the uplift is mainly horizontal rather than vertical

tomographical mapping of the lithosphere and asthenosphere beneath southern scandinavia and adjacent areas abstract in this paper we report on a seismic tomographical survey of the lithosphere asthenosphere beneath southern scandinavia and adjacent areas the ach inversion technique has been used and the observational data in the form of p wave travel time residuals have been obtained from seismogram records from networks in denmark southern norway and southern sweden the volume under investigation subdivided into four levels layers of 7 7 knots blocks each for estimation of velocity perturbations covers the area 54 62 n and 7 20 e and depth range 0 600 km major findings are as follows level 1 0 100 km pronounced velocity lows beneath southern jutland zealand the variscan foredeep and the sveco norwegian province of southeastern norway 1500 ma old the svecofennides of southeastern sweden exhibit relatively high velocities while the sveco norwegian province of southwestern sweden 1700 ma old is characterized by moderately high velocities the transition between velocity highs and lows coincides with the tornquist teisseyre line ttl or fennoscundian border zone level 2 100 300 km velocity anomalies are less pronounced at this level although velocity lows are found beneath the northern jutland zealand area and slightly northwestwards of the oslo rift the only velocity high is found beneath the southeasternmost part of sweden the ttl does not appear to have a clear counterpart in the velocity anomaly pattern in this depth range although a clearly unambiguous answer here would require far more extensive observational data a research goal of the future level 3 300 500 km and level 4 500 600 km velocity anomalies are weak which is not unexpected for the upper mantle beneath shield areas