palaeomagnetism of the late precambrian vadso and barents sea groups varanger peninsula norway abstract a palaeomagnetic study has been made on neoproterozoic rocks from the varanger peninsula northernmost norway several of the formations studied contain a stable magnetization of diagenetic origin two main groups of directions were obtained one a with a steep southeasterly positive direction the other b with a shallow northern direction the group a direction is not in accordance with the stratigraphic age of the rocks and the fennoscandian apparent polar wander path hence it may reflect a remagnetization the group b direction yields a late riphean early vendian palaeopole at 24 n 207 e dp 5 dm 11 n 11 in good agreement with data from contemporary rocks of the kola peninsula the varanger peninsula is transected by the trollfjord komagelv fault which divides the peninsula into the allochthonous barents sea region and the autochthonous varangerfjord region previous palaeomagnetic studies have suggested that the barents sea region has moved 500 to 1000 km to its present position this study indicates that the movement has been less than 250 km which is in better agreement with geological information

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

palaeomagnetism mafic dykes and the protogine zone southern sweden the protogine zone pz forms the border between the svecofennian and sveconorwegian domains of southern sweden intruded in the zone are three generations of mafic dykes dated 1510 ma 1180 ma and 930 ma a palaeomagnetic study of these dykes in the central and southern parts of the pz yields two main directions of magnetization a d 291 i 78 n 31 and b d 129 i 40 n 13 these directions give palaeopoles on the sveconorwegian loop of the fennoscandian apparent polar wander path apwp a third poorly defined group c yields a mean declinationof 185 and inclination of 9 with the corresponding palaeopole situated on the c 1500 1600 ma part of the apwp two other sites yield palaeopoles located in the 1200 ma part of the path the data imply that the sveconorwegian orogeny caused a remagnetization of the two older dyke generations and that this remagnetization was complete in the southern part of the pz while a few dykes retained their original magnetization in the central part of the zone either the difference between the a and b directions record appreciable apw movement during differential cooling of the pz or the a and b directions may represent an asymmetric reversal of the earth s magnetic field in the latter case they may be contemporary and have implications for the size and form of the sveconorwegian loop of the fennoscandian apwp

global correlation of late cenozoic fluvial deposits a synthesis of data from igcp 449 abstract data on fluvial sequences from around the world collected as part of igcp 449 has allowed comparison between such records with the purpose of searching for meaningful patterns a key finding has been that fluvial incision rates as measured by the vertical separations of well dated terrace deposits one from another and from the modern valley floor which can be used as a proxy for uplift are often very similar in different parts of the world but vary significantly between systems on different types of crust the collection that this paper introduces coupled with previous igcp 449 publications cited confirms that the aim of a worldwide compilation of fluvial archives has been achieved with a number of syntheses already published

seismic and geoelectric evidence for collisional and extensional events in the fennoscandian shield implications for precambrian crustal evolution extensive deep seismic and electromagnetic sounding programs have resulted in tens of deep refraction seismic profiles normal incidence reflection profiles several hundred magnetotelluric soundings and several magnetovariational array observations results are compiled into four maps 1 moho depth 2 thickness of the lower crustal high velocity layer vp 7 km s 3 depth to the upper interface of the high velocity layer and 4 distribution of upper and middle crustal electrical conductors the moho depth is approximately 40 km in the archaean domain and varies between 40 and 65 km in the svecofennian domain most of the thickness variation 0 24 km is concentrated in the lower crustal high velocity layer electrically the crust is formed of several rather homogeneous either resistive or conductive blocks that are bounded by upper and middle crustal conductive zones the cratonized archaean crust experienced an extensional phase during which a thin mafic lower crustal high velocity layer was produced in association with mafic dykes 2 2 2 1 ga and well conducting volcanogenic and metasedimentary belts 2 5 2 0 ga the moho of the archaean domain was consequently reformed in the early proterozoic major thickening of the crust 50 65 km occurred when a svecofennian island arc and the archaean craton collided the svecofennian orogeny 1 76 1 9 ga resulted in a collage of metasedimentary rocks squeezed between crustal blocks of the island arc the internal terrane boundaries are seen as good reflectors and or inclined or vertical conductors that extend at least to the middle crust the thick high velocity 7 km s lower crust is a combination of mafic lower crust of the island arc crustal blocks and mafic additions from either delamination of the lithosphere after the collision or from a period of late svecofennian subduction the collisional boundary is also preserved as a discontinuous conductive zone between the archaean and svecofennian domains the thickened crust was subsequently thinned to 42 km along e w striking zones during an extensional period characterized by the intrusion of anorogenic rapakivi granitoid batholiths coeval mafic dykes and gabbro anorthosite bodies 1 6 1 5 ga the crust of the central part of the shield has since remained virtually intact

electromagnetic images of modern and ancient subduction zones abstract one question frequently posed since the advent of plate tectonic theory is when did it begin correlations of zones of enhanced electrical conductivity with orogenic belts dated to half the age of the earth support the suggestion that plate tectonic processes have been in operation since at least the early proterozoic the causes for such zones are examined and whereas anomalies associated with modern and recent subduction collision systems can be readily explained an explanation for palaeozoic and proterozoic ones is more difficult correlation of geophysical characteristics associated with the trans hudson north america and svecofennian fen noscandia orogenic zones and consideration of the paleopole positions of those zones and the superior and slave provinces during the period 1 900 1 825 ga lead to the suggestion that a pan scandamerican orogenic zone was in existence at that time

paleomagnetic constraints on the crustal evolution of the yangtze block southeastern china abstract paleomagnetic poles for the yangtze block ranging in age from early cambrian to quaternary are compiled up to the end of 1989 the data are classified into a b c or d quality categories by applying a specific reliability grading scale a filtering technique is then used to separate out the less reliable from the more reliable poles unit weight is finally assigned to the remaining results to establish an apw path for this block the new apw path reveals two sharp hairpins in the permian jurassic apw segment which were not observed in previous apw paths the first hairpin occurs around the permian triassic boundary which may be related to the rapid subduction of the qinling ocean beneath the sino korean block the second one occurs around the triassic jurassic boundary corresponding to the late indosinian orogeny in addition the early paleozoic path roughly describes an anticlockwise self closing loop which is opposite in sense to that found in the apw path for gondwana land and implies that the yangtze block had been separated from gondwana land since that time the crustal evolution for the yangtze block is presented in terms of paleolatitude and paleorotation during the paleozoic and the early mesozoic the block was located near the equator after the late permian the block underwent considerable poleward motion until it approached its present relative latitudinal position and orientation in the cretaceous relative motions of the yangtze block with respect to eurasia are discussed by comparing their apw paths showing that the yangtze block was not part of the eurasian continent until the cretaceous and that since then no substantial motions occurred between them

the lower middle ordovician palaeofield of scandinavia southern sweden revisited abstract we present a palaeomagnetic pole for the arenig llanvirn swedish orthoceras limestones based on a new analysis of some earlier reported palaeomagnetic data this new pole 18 n and 046 e differs from the original estimate of 30 n and 46 e the revised pole probably represents the most reliable ordovician pole from baltica although some uncertainties remain concerning the relative importance of diagenetic depositional controls on remanence acquisition based on the new pole from the swedish orthoceras limestone and recent palaeomagnetic results from the northern scandinavian caledonides it is now evident that baltica lay within southerly latitudes of between 30 and 60 s in cambro ordovician times subsequent anticlockwise rotation relative to eastern avalonia southern britain may indicate dextral shear across the tornquist suture prior to mid late ordovician amalgamation of eastern avalonia and baltica

tectonics of the kola collision suture and adjacent archaean and early proterozoic terrains in the northeastern region of the baltic shield as preparation for the deep seismic and other geophysical experiments along the polar profile which transects the granulite belt and the kola collision suture structural field work has been performed in northernmost finland and norway and published geological information including data from the neighbouring soviet territory of the kola peninsula have been compiled and reinterpreted based on these studies and a classification according to crustal and structural ages the northeastern region of the baltic shield is divided into six major tectonic units these units are separated and outlined by important low angle ductile shear or thrust zones of late archaean to early proterozoic age the lateral extension of these units into soviet territory and their involvement in large scale crustal deformation structures are described using the view down the plunge method a generalised tectonic cross section that predicts the crustal structures along the polar profile is compiled and the structures around the kola deep drill hole are reinterpreted the kola suture belt through parts of which the kola deep bore hole has been drilled is considered to represent a ca 1900 ma old arc continent and continent continent collision suture it divides the northeastern shield region into two major crustal compartments a northern compartment comprising the murmansk and sorvaranger units and a southern compartment including the inari unit the granulite belt and the tanaelv belt as well as the more southernly situated south lapland karelia craton of the karelian province of the svecokarelian fold belt the kola suture belt is outlined by a 2 40 km wide and ca 500 km long crustal belt composed of 1 1 early proterozoic ca 2400 2000 ma old metavolcanic and metasedimentary sequences which originally formed part of the attenuated margin of the northern archaean compartment and 2 2 the remains of a ca 2000 1900 ma old predominantly andesitic island arc terrain this island arc terrain was built up above a sw plunging subduction zone initiated ca 2000 ma ago in the southern part of a newly formed oceanic domain the kola ocean due to continued subduction and complete consumption of this ocean the northern passive margin deposits and the island arc terrain were brought into tectonic juxtaposition and during the final arc continent and continent continent collision they were overthrusted onto the northern archaean continent along its southern boundary the kola suture belt is tectonically overlain by the archaean rocks of the inari unit this unit was derived from a microcontinent split from the southern compartment the depositional basin of the protoliths of the granulite belt being formed to the south of the microcontinent the inari microcontinent appears to have wedged out towards the southeast as the continuation of the granulite belt north of the white sea is in direct tectonic contact with the kola suture belt the granulite belt is composed of high grade paragneisses and minor amounts of meta igneous rocks the paragneisses formed from thick turbidite and mass flow deposits lain down in a back arc basin south of the inari microcontinent a thermal anomaly beneath the partly oceanic basement of the back arc basin is believed to have contributed to the ca 2000 1900 ma old granulite facies metamorphism of the granulite assemblages granulite facies conditions still prevailed when the inari microcontinent overrode the granulites and when the granulite belt as such was formed and was overthrusted for at least 100 km towards the southwest in conjunction with the latter event the rocks of the basement of the basin also became involved in thrust movements these now form the tanaelv belt which shows gradational tectonic contacts towards underlying cover and basement rocks of the south lapland karelia craton although not all parts of this craton were affected by the svecokarelian deformation it is considered to belong to the karelian province of the svecokarelian fold belt a ca 1900 1800 ma old episode of wrench faulting and the intrusion of 1790 1770 ma old post kinematic granites concluded the svecokarelian evolution of the northeastern shield region

minimum continental velocities with respect to the pole since the archean abstract paleomagnetic apparent polar wander paths provide a method for estimating minimum plate velocities which can be extended much further into the geologic past than can be estimated based on sea floor magnetic anomalies minimum velocities can be determined from the rate of change of latitude as derived relative to a fixed pole such latitudinal velocities have been determined for the center of mass of the north american baltic siberian and african shields since the archean the results indicate that plate velocities in the past have at times easily exceeded those for present day continents and that they often were equivalent to present day oceanic velocities although there are peaks and troughs through time north american velocities are significantly greater prior to one billion years ago than those of siberia or baltica

geological and geophysical studies of the agoudal impact structure central high atlas morocco new evidence for crater size and age since the discovery of shatter cones scs near the village of agoudal morocco central high atlas mountains in 2013 the absence of one or several associated circular structures led to speculation about the age of the impact event the number and the size of the impact crater or craters additional constraints on the crater size age and erosion rates are obtained here from geological structural and geophysical mapping and from cosmogenic nuclide data our geological maps of the agoudal impact site at the scales of 1 30 000 6 km 2 and 1 15 000 2 25 km 2 include all known occurrences of scs in target rocks breccias and vertical to overturned strata considering that strata surrounding the impact site are subhorizontal we argue that disturbed strata are related to the impact event three types of breccias have been observed two of them br1 2 and br2 could be produced by erosion sedimentation consolidation processes with no evidence for impact breccias while breccia br1 might be impact related the most probable center of the structure is estimated at 31 degrees 59 13 73 n 5 degrees 30 55 14 w using the concentric deviation method applied to the orientation of strata over the disturbed area despite the absence of a morphological expression the ground magnetic and electromagnetic surveys reveal anomalies spatially associated with disturbed strata and sc occurrences the geophysical data the structural observations and the area of occurrence of scs in target rocks are all consistent with an original size of 1 4 4 2 km in diameter cosmogenic nuclide data cl 36 constrain the local erosion rates between 220 22 m ma 1 and 430 43 m ma 1 these erosion rates may remove the topographic expression of such a crater and its ejecta in a time period of about 0 3 1 9 ma this age is older than the agoudal iron meteorite age 105 40 kyr this new age constraint excludes the possibility of a genetic relationship between the agoudal iron meteorite fall and the formation of the agoudal impact site a chronolgy chart including the atlas orogeny the alternation of sedimentation and erosion periods and the meteoritic impacts is presented based on all obtained and combined data

long term fluvial archives in hungary response of the danube and tisza rivers to tectonic movements and climatic changes during the quaternary a review and new synthesis abstract the paper discusses the quaternary evolution of the danube and tisza rivers and their main tributaries in the context of evolution of the entire pannonian basin which is europe s largest intramontane basin within the alp carpathian orogen the palaeo drainage reconstruction of the pannonian basin for the pre quaternary period is outlined in connection with the gradual regression of lake pannon since the late miocene deltas of rivers that entered the basin from the northwest and northeast were gradually transformed into extended alluvial plains thus the earliest possible ancestor of the danube coming southeastwards from the alps could be as old as late miocene by the pliocene the whole lake pannon was infilled the former extensional basin formation was replaced by a compresional stress field which resulted in an uplift of the marginal flanks and late stage subsidence anomalies the increasing relief led to the formation of the quaternary drainage pattern the actively subsiding young basins were filled by fluvial sediments transported by the danube and tisza river systems from the uplifting mountains between the subsiding regions of the little and great plains the danube has formed an antecedent valley with terrace staircases between the uplifting sections of the transdanubian range and the north midmountains the formation of the terraces is attributed to periodic climate changes during the pleistocene combined with differences in the uplift rate the paper gives a complex overview of the classical chronology of the six terraces based on various data sources mostly dating of loess paleosol sequences travertines aeolian sand and tephra strata overlying the fluvial sediments complemented by scattered vertebrate faunal data and archaeological evidence directly from the terrace sediments the quaternary drainage pattern evolution of the great plain with a strong tectonic control is discussed in detail rivers originating from the uplifting marginal areas were drawn towards the subsiding depressions which served as local base level changes in subsidence rates in space and time throughout the quaternary resulted in the evolution of a complex drainage pattern a special emphasis is placed on the late pleistocene holocene development of the middle tisza region and the koros basin where the berettyo koros rivers form an eastern tributary system of the tisza river a comparative evaluation of these two areas is especially relevant as they provide insights into large scale late pleistocene avulsion of the tisza river osl dating complemented with inferred transport directions determined from heavy mineral analysis of fluvial sediments in the koros basin has revealed an ancient large meandering river system that can be identified with the palaeo tisza which was flowing along a tectonically controlled depression during the late pleniglacial successions in the middle tisza region have allowed differentiation between the older channels of the palaeo bodrog river and the sajo hernad alluvial fan and the younger meander belts of the new course of the tisza in the tisza system changes in river style braided to various scales of meandering show correspondence to millennial scale climate changes of the last 25 ka while in the koros basin the effects of tectonics are overprinted onto the regional climatic signals

a practical suggestion regarding the use of scandinavian and russian palaeomagnetic data to determine the palaeo position of baltica in ordovician time abstract there is a discrepancy between ordovician palaeomagnetic pole positions from the russian plate and from scandinavia however it is clear from faunal and tectonic evidence that the two regions were parts of a single continent through palaeozoic time data from both regions are few in number and some are controversial in nature therefore it is suggested that the palaeomagnetic discrepancy merely reflects the incomplete nature of the palaeomagnetic dataset for baltica it is notable that palaeolatitudes for baltica derived from the scandinavian dataset are confirmed by other lines of geological evidence it is therefore concluded that the scandinavian dataset offers the most reliable palaeomagnetic controls on the lower palaeozoic drift history of baltica placing it in mid southerly latitudes in early ordovician time rotated clockwise through at least 50 relative to its present orientation

palaeomagnetism magnetic fabrics and the structural style of the hornelen old red sandstone western norway the old red sandstone of the hornelen district western norway forms part of a major fold terrane and the principal magnetization is associated with the late devonian to early carboniferous svalbardian solundian orogeny the present fault margins of the hornelen massif claimed by various authors to represent subordinate devonian splay faults associated with megashearing in the north atlantic or lateral ramps in a regional extensional tectonic setting are shown to be post devonian structures and the present outline of the hornelen massif is a mesozoic jurassic graben

palaeomagnetic studies of proterozoic rocks from the lake onega region southeast fennoscandian shield summary in a recent compilation elming et al 1993 presented a new apparent polar wander path for the proterozoic of the fennoscandian shield however only a minor portion of the data used for this and for the earlier compilations has been obtained from the eastern part of the fennoscandian shield thus prompting the present study in this paper we present new palaeomagnetic data for the lake onega region the sections studied comprise quartzites as well as extrusive and intrusive rocks of vepsian 1900 1650 ma and jatulian 2300 2100 ma ages which represent svecofennian and jatulian tectonomagmatic events respectively thermal and alternating field demagnetization experiments reveal the presence of several components of the natural remanent magnetization of normal and reversed polarity component an has been identified in a sill of vepsian gabbro dolerites 1770 ma exposed at rybreka locality and yields a palaeopole at 37 8dn 210 6de dp 4 0d dm 7 7d this magnetization is interpreted to be of thermoremanent origin in the vepsian shoksha quartzite ca 1800 ma cut by the sill and in the underlying basalts the palaeopoles indicated by the presumably primary components an and ar are at 45 1 dn 206 4de dp 3 6d dm 6 5d and 44 2dn 241 5de dp 6 1d dm 9 6d respectively component an was isolated in the jatulian basaltic lavas ca 2200 2300 ma at girvas locality also however in this case it appears as secondary and yields nearly the same palaeopole 43 1dn 211 5de dp 4 9d dm 8 8d component br was isolated in vepsian gabbro dolerites and quartzites sampled from zones altered by monzonite veins representing the final phase of sill emplacement this component yields a palaeopole at 13 9dn 171 3de dp 2 4d dm 4 8d for gabbro dolerites and at 10 5dn 201 8de dp 8 4d dm 14 9d for quartzites the vepsian and jatulian rocks also show the presence of presumably younger 1500 ma magnetization components c d and e the presumably primary stable dual polarity components determined in the jatulian lavas and interlayers of the red siliceous rocks yield a palaeopole at 0 8ds 245 4de dp 7 2d dm 11 6d for the upper part of the section component f and a palaeopole at 36 0ds 307 4de dp 9 9d dm 14 9d for the lower part of the section component g these palaeomagnetic poles are compared with the apparent polar wander path of fennoscandia for the interval 2400 1200 ma after elming et al 1993 and with some recent results from vepsian and jatulian rocks

scaling relationships in intraplate fracture systems related to red sea rifting abstract the geometry of fracture systems was analysed at various scales in the western arabian sedimentary cover rocks investigations covered a wide range of scales from regional surveys to outcrops in well exposed areas and were based on the field mapping of fractures and the interpretation and digitizing of fracture patterns on aerial and satellite images as a first step a ground structural analysis was carried out which provided a better understanding of the relationships of joint and fault systems related to red sea rifting secondly fracture datasets were subjected to statistical and geostatistical analyses of direction size spacing and density and the variability in these quantities with space and scale the main result of this combined approach is that fracturing is not a self similar process from the centimetre scale up to the 1000 km scale in this case the spatial organization of joints and faults appears to be highly controlled by both the red sea extensional regime and the thickness of the different mechanical layers that constitute the crust at different scales i e sedimentary beds basins and the seismogenic crust

absolute reconstructions of the paleozoic oceans abstract the orientation of three intraplate magmatic belts the mongolian from 130 to 280 m y south siberian from 320 to 400 m y and baltic from 365 to 400 m y belts which can be considered as hot spot tracks are used together with apparent paleomagnetic pole wander paths to reconstruct true motions of continents during paleozoic times the reconstructions obtained show that the old late precambrian pangea continued to exist in the early paleozoic with an inner arrangement of constituent continents strongly different from that of the late paleozoic pangea the continents were constantly located in the eastern hemisphere only the western hemisphere was occupied by the paleo pacific oceans four oceans existed in paleozoic times between the continents in the eastern hemisphere 1 the iapetus which originated in the late precambrian and closed before devonian times 400 m y 2 the asiatic paleo ocean which originated in the late precambrian and closed 450 m y ago 3 the uralian paleo ocean which opened around 500 m y ago and closed in the latest permian 240 230 m y 4 the paleo tethys which opened in the ordovician 480 450 m y and became the mesozoic tethys life duration of the oceans was 200 400 m y the main trend in the earth s evolution during the paleozoic was a break up of the old precambrian supercontinent and construction of the new late paleozoic pangea

description of the polar profile transect display abstract the polar profile transect display attempts to integrate all the existing aeromagnetic gravimetric geological and geochemical investigations with the results of the deep seismic and electromagnetic studies carried out along the polar profile our description is based on the stage of joint interpretation reached at the second earth science study centre the nearly 440 km long ne sw trending polar profile traverses archaean and early proterozoic parts of the baltic shield the thickness of the crust varies between 40 and 50 km and consists of three distinct crustal layers a geological interpretation based on the various geophysical results is given for the uppermost 10 14 km thick layer in the central part of the profile the overthrust early proterozoic lapland granulite belt forms a wedge shaped body reaching the middle crust the upper crust is underlain by a transparent layer 16 18 km thick with a smooth increase in velocity and density a reflective lowermost 10 21 km thick layer has a high velocity gradient the crust mantle has a pronounced topography

thermal evolution of the earth abstract models to study the thermal evolution of the earth have been developed based on a calculation of finite amplitude thermal convection in an incompressible and viscous mantle with temperature and pressure dependent physical parameters the lateral dependences of the variables are resolved through their spherical harmonic representations while the radial and time dependences are calculated using numerical procedures the major emphasis of this paper is to illustrate the effects of different factors on the thermal evolution of the earth rather than to present a definite earth model the main conclusions achieved in this study are 1 1 a significant portion of the present day temperature in the mantle and surface heat flux is due to initial heat of the earth resulting from accretion and core formation 2 2 since the effective rayleigh number of the earth s mantle is high solid state convection is found to be sufficiently efficient to maintain an almost adiabatic temperature gradient within the convecting region while large temperature gradients exist across the thermal boundary layers core mantle boundary and below the lithosphere 3 3 the convection is oscillatory with avalanche type properties this is due to the slow development of instability in the thermal boundary layer near the surface which causes the episodic behavior the periods of the oscillations are about 50 250 m y the oscillation in the mantle also induces periodic features in the surface heat flux and the thickness of the lithosphere mantle viscosity structure plays an important role in the cooling rate and nature of oscillations 4 4 the mobility of the lithosphere is an important factor controlling the thermal state of the earth s interior a mobile lithosphere will cool the interior faster

seismic structure and composition of the crust beneath the southern scandes norway abstract new results on p and s wave seismic velocity structure in southern norway indicate that the crust has an average poisson s ratio of 0 25 is predominantly of felsic intermediate composition and lacks a significant mafic lower crust a crustal scale refraction seismic study magnus rex mantle investigations of norwegian uplift structure refraction experiment acquired data along three 300 to 400 km long active source seismic profiles across the southwest scandinavian domain in southern norway the youngest section of the fennoscandian shield moho depths in the domain are 36 40 km thinning towards the continental shelf and oslo graben the high vp lower crust beneath the southwest scandinavian domain vp 7 km s is around 4 km thick crustal structure in the adjacent svecofennian domain differs significantly moho depths reach 50 km and an up to 24 km thick high vp lower crust is present strong p and s wave arrivals are recorded from magnus rex from which poisson s ratio for the crust is calculated the unusually strong s wave arrivals allow rare insight into crustal poisson s ratio structure that is not commonly available from active source data the thinness of the mafic lower crust in southern norway is in significant contrast to other parts of the fennoscandian shield and delamination or convective removal of the lower crust is proposed tectonic events that may have triggered detachment of the lower crust include extensional collapse after the caledonian orogeny and permian extension

heat flow and thermal structure of the lithosphere across the baltic shield and northern tornquist zone abstract terrestrial heat flow data from archean and proterozoic provinces of the baltic shield its southwestern margin and the tectonic transition to the younger tectonic units of central europe northern segment of the european geotraverse are compiled and examined for local and regional variations a 2 d numerical lithospheric temperature and heat flux model is presented covering 1800 km of the deep seismic fennolora profile across the baltic shield and 500 km of eugeno s northern eugemi seismic lines across the northern tornquist zone and the danish and north german basins thermal lithosphere defined as the outer layer with relatively low temperatures t c in which heat transfer is dominantly by conduction is found to vary markedly in thickness from 85 110 km beneath basins and tornquist zone to about 150 km in central shield areas and 200 250 km in areas of low heat flow along the northern shield profile these results are in very good agreement with seismological information from surface wave dispersion analysis and the refraction wide angle seismic interpretation of fennolora data the combined thermal and seismologic evidence indicates that the uppermost mantle region stratified in seismic velocity is restricted to the upper relatively cold thermal layer which is interpreted to constitute shield lithosphere local and to some extent regional variations in surface heat flow are found to be closely related to variations in near surface radiogenic heat production for the southern baltic shield a heat flow heat production relationship of q 0 32 8 7 6 h 0 is derived regional increase in heat flow from northern and central shield provinces to the shield margin and the tornquist zone is interpreted to be related partly to differences in upper crustal heat generation partly and in southernmost areas mainly to an increased heat flux from the upper mantle upper mantle heat flux is estimated at 20 25 mw m 2 along the northern and central shield transect increasing to 30 40 mw m 2 beneath the shield margin basins and tornquist zone lithospheric thickness variations in this region of northern europe are found to be closely associated with variations in heat flux from the uppermost mantle and perhaps to a large extent controlled by geodynamically determined lateral differences in upper mantle heat flux earthquake focal depth data and information on crustal strength show baltic shield seismicity to be limited generally to the upper 20 25 km brittle part of the crust at temperatures less than 300 400 c maximum source depth of magnetic anomalies at temperatures near 600 c is in the deepest part of the crust near the crust mantle boundary in the southern region about 25 km depth along the tornquist zone and about 35 40 km in southern shield areas and in the uppermost mantle 60 70 km depth in the northern shield areas some petrologic implications are discussed the present deep crust is supposed to be at lower temperature pressure conditions than at early stages of formation however also the present deepest parts of the shield crust 35 40 km are within the stability field of eclogites the general absence of significant gravity anomalies in areas of marked crustal thickness variations indicate high density eclogites to be present in significant amounts in the deep shield crust

lower proterozoic glaciogenic deposits eastern finland in the lower proterozoic sariolian group of eastern finland the metasedimentary urkkavaara formation possesses the attributes of a glaciogenic deposit within the formation four members have been established a lower siltstone argillite member a graded sandstone member an upper siltstone argillite member and a diamictite member the lowest three members contain lonestones the upper siltstone argillite member passes gradually over 1 m into a massive diamictite many lonestones in the siltstone argillite members are clearly dropstones these members and their association with diamictite constitute strong evidence for a glaciogenic origin for the formation the vertical transition of siltstone argillite into diamictite indicates a similar raining down mechanism of deposition for the diamictite the thickness of the formation 30 to 60 m minimum suggests deposition from icebergs rather than from seasonal ice for the laminated members and the diamictite the graded sandstone member is interpreted to be a result of turbidity current deposition near a glacial terminus a glaciomarine environment is suggested lonestone bearing units and diamictites in the soviet union just east of finland have recently been reinterpreted as glaciogenic deposits there appears therefore to have been an early proterozoic continental glaciation about 2 500 to 2 300 m y ago on the baltic shield approximately the same time that early proterozoic glaciation occurred in north america

palaeomagnetism of precambrian rocks in northern sweden and its correlation to radiometric data abstract a palaeomagnetic study has been performed on palaeo to mesoproterozoic basic intrusions and volcanic rocks from the fennoscandian shield in northern sweden three possibly four different generations of magnetizations were identified the oldest assigned to a svecofennian age 1 86 1 89 ga a second generation is related to the intrusion of granitoids of 1 80 1 76 ga in this geological event probably also the third group of directions has its origin these different magnetizations may indicate that there are at least two different generations of basic intrusions in northern sweden a fourth group of directions is isolated as overprints this magnetization is interpreted to be a subjotnian magnetization reflecting a previously unrecognized regional subjotnian metamorphic event in the northwestern part of the fennoscandian shield the drift history for the fennoscandian shield during the period 1 88 1 50 ga has been defined based on these new palaeomagnetic data

palaeomagnetism of the lappajarvi impact structure western finland abstract palaeomagnetic and rock magnetic results are presented for rocks from the lappajarvi meteorite impact structure western finland oriented samples were collected from the melt rocks in the central island of the crater and of the palaeoproterozoic svecofennian target rocks at the crater rim the natural remanent magnitization nrm of the melt rocks is generally weak and metastable during demagnetization the characteristic nrm component carried by pyrrhotite yields a palaeomagnetic pole latitude 55 4 n longitude 152 6 e dp 6 2 dm 8 7 dg 6 sites suggesting an age of 195 ma for the impact the results from a deep core drilled from the impact melt rock display similar palaeomagnetic directions but with slightly shallower inclinations the palaeomagnetic age differs from the 40ar 3939ar age of 77 ma for the impact assuming that the 40ar 39ar age is correct the discrepancy between palaeomagnetic and radiometric ages is best explained by post impact and post cooling tilting of the melt layer the tilting 15 may be due to isostatic rebounds along joints in the melt impact remanence has also been isolated in a few target rocks at the rim this poorly defined overprint could be a mild shock remanent magnetization superimposed on the 1 9 ga old svecofennian direction this interpretation is supported by the observation that the nrm intensity is enhanced in specimens having the impact overprint the palaeomagnetic results are consistent with earlier interpretations based on geophysical morphological mineralogical and geochemical data that the lappajarvi structure is a complex meteorite impact site less than 200 ma old

holocene floodplain sediment storage and hillslope erosion within the rhine catchment the response of fluvial systems to land use and climate change varies depending on catchment size while forcing response mechanisms of small catchments are reasonably well understood the response of larger drainage basins is less clear in particular the impact of land use and climate change on the rhine system is poorly understood because of the catchment size 185 000 km 2 and the long history of human cultivation which started approximately 7500 years ago a sediment budget is calculated to specify the amount of alluvial sediment that was deposited during the holocene and to estimate long term soil erosion rates the results suggest that 599 14 10 9 t of holocene alluvial sediment is stored in the non alpine part of the rhine catchment south and central germany eastern france the netherlands about 50 of holocene alluvial sediment is deposited along the trunk valley and the delta upper rhine lower rhine coastal plain while the rest is stored along the tributary valleys the floodplain sediment storage corresponds to a mean erosion rate of 0 559 0 16 t ha per yr 38 59 10 7 mm kyr across the rhine catchment outside the alps this holocene averaged estimate amounts for sediments that were delivered to the channel network and is at the lower limit of erosion rates from other studies of different methodology

tectono magmatic control on vertical dip slip basement faulting an example from the fennoscandian shield vertical dip slip basement faults play an important role in the evolution and structuring of the earth s crust the proterozoic anorogenic rapakivi anorthosite setting of the fennoscandian shield in southern finland exhibits a widespread pattern of vertical dip slip basement faults that are deeply eroded the porkkala mantsala pm fault located c 30 km w of helsinki is part of a system of crustal lineaments that closely follows the outcrop pattern of mid late proterozoic anorogenic crustal elements such as basic dyke swarms the outline of rapakivi granites and remnants of sediment filled grabens these lineaments are formed by low grade dip slip faults that overprint svecofennian shear zones structural analysis of the pm fault supports an interpretation in terms of reactivation of a high grade ductile wrench zone successive stages of brittle deformation visible in as well the pm fault and the obbnas granite demonstrate that brittle deformation in the pm fault is coeval with the intrusion of the anorogenic obbnas rapakivi granite based on the spatial and temporal relationship of anorogenic magmatism and block faulting a genetic relationship is proposed

palaeomagnetism of middle proterozoic basic intrusives in central sweden and the fennoscandian apparent polar wander path abstract palaeomagnetic results are presented from three sets of post svecokarelian intrusions in central sweden the first is a system of e w trending doleritic dykes including the large breven and hallefors dykes and with a probable emplacement age of c 1530 ma their original nrm is disturbed by later events but two magnetic directions group ew1 with decl 359 5 incl 6 2 95 8 2 n 35 and group ew 2 with decl 11 1 incl 51 3 95 10 1 n 14 have been isolated and give palaeopoles at 27 7 n 196 8 e and 61 7 n 176 1 e respectively the group ew1 direction is considered to represent the original emplacement nrm this is based on comparison with contemporary palaeomagnetic data from elsewhere in fennoscandia the second set comprises dolerites and quartzporphyries the tuna dykes age 1371 ma they present a stable magnetization with decl 14 8 incl 18 1 95 9 6 and a palaeopole at 21 1 n 180 4 e the third set studied are dolerites belonging to the dala and jamtland complexes of the central scandinavian dolerite group csdg age c 1250 1200 ma post jotnian they present well defined magnetic directions that give palaeopoles close to the equator around 155 e in agreement with other published palaeomagnetic csdg data however there are deviating results obtain from dolerites belonging to the dala complex of the csdg those deviating have a mean decl of 12 9 incl 1 4 95 18 1 n 5 which gives a palaeopole at 27 0 n 179 5 e situated within the area of the 1600 1500 ma poles results from three probably sweconorwegian c 1000 850 ma doleritic dykes are also presented the results obtained are compared with other contemporary palaeomagnetic poles from fennoscandia and a revised apparent polar wander path is presented and discussed

paleomagnetism of 2 44 ga mafic dykes in russian karelia eastern fennoscandian shield implications for continental reconstructions abstract results of a paleomagnetic study of paleoproterozoic dykes and archean host rocks in the eastern fennoscandian shield are reported here most of the dykes are related to ca 2440 ma layered intrusions and have isotopic ages between 2349 and 2476 ma the dykes form an extensive nw se and e w trending swarm the majority are unaltered gabbronorites which carry stable remanent magnetizations with multicomponent nature the high temperature remanence component d which is thought to represent the primary 2 44 ga magnetization of the dykes has a paleomagnetic pole at 20 s 278 8 e a95 6 0 n 11 sites another d like component d with shallower inclination vgp 9 6 n 256 2 e eight samples at one site from baked archean host rock may also represent the primary remanence of the dykes the commonest remanence component a records an extensive svecofennian partial overprint and is interpreted to be ca 1 84 ga old it yields a paleomagnetic pole at 52 6 n 226 8 e a95 2 7 n 23 the fourth component b has a paleopole at 53 9 n 169 6 e a95 6 3 n 16 and is inferred to be ca 1 75 ga on the basis of isotopic dating although ages of 2 1 1 3 and 0 3 ga are also possible baked contact tests for the 2 44 ga dykes are negative as shown by the occurrence of component d in archean host rocks beyond one dyke width it is proposed that the negative baked contact test indicates regional reheating and remagnetization of the archean basement at ca 2 44 ga ago due to the spatially vast occurrence of coeval 2 44 ga mafic dykes layered intrusions volcanics and granites in the area based on this hypothesis it is suggested that component d in the dykes may be primary and acquired at ca 2 44 ga ago the similarity of pole d with poles from coeval dykes and layered intrusions elsewhere in the karelian province and kola peninsula suggest that within the archean domain of the fennoscandian shield no large scale block movements have taken place since 2 44 ga at 2 44 ga the fennoscandian shield was located at the latitude of ca 30 s which is different to that at 1 84 and 1 75 ga when fennoscandia was located at the latitudes of ca 30 n a continental reconstruction based on pole d and the pole from the 2 45 ga matachewan dykes of the superior craton in laurentia places the kola peninsula adjacent to southern greenland reconstructions based on recent geological interpretations and a reconstruction yielding pole d are also discussed

the svecofennian orogen a tectonic model abstract recent geological studies have provided new concepts concerning the timing of the final break up of the archaean craton age data on detrital zircons from sedimentary rocks in sweden and finland show marked similarities and indicate pervasive homogenization of accumulated material geochemical and isotopic studies in sweden and finland suggest the existence of two volcanic belts separated by an area dominated by sedimentary rocks however the ages of metamorphism and plutonism differ in particular the late post 1 85 ga granitoid magmatism was more voluminous in sweden than in finland the svecofennian crust in finland is anomalously thick up to 65 km and is characterized by a high velocity lower crust of variable thickness the evolution of the svecofennian orogen is modelled by opening of the pre svecofennian ocean at 1 95 ga and progressive accretion of two arc complexes to the archaean craton between 1 91 1 87 ga initial accretion at 1 91 ga led to thrusting of the slices of an island arc complex and proterozoic marine strata onto the thickening craton margin subduction reversal with simultaneous anti clockwise rotation of the converging plate or convergence of another plate resulted in subduction below the newly accreted arc complex the tectonic style at the craton margin changed from thrusting to lateral displacement in the east in finland in the west in sweden the subduction zone retreated along a transform fault a large sedimentary basin closed in the east 1 89 ga ago when another arc complex collided against the accreted one collision resulted in lithospheric thickening subsequent delamination of the thickened lithosphere and compensation by a hot mantle underplate and large scale granitoid magmatism as a result of mixing between mantle magma and remelted arc complex material the thick dense layer that formed in the lower crust as the result of underplating kept the thickened crust in isostatic balance thus the exhumation rate in this area was similar to the rate in areas of thinner crust subduction and sediment accumulation continued in the west until at least 1 87 ga when large areas in the east were already stabilized an intracratonic transpressional zone developed as the result of continued convergence subsequent extensional collapse along the transpressional zone caused melting of svecofennian sedimentary rocks migmatization and generation of the 1 84 1 82 ga anatectic granites of southern finland and central sweden the voluminous post 1 85 ga granitoid magmatism in sweden is tentatively associated with an east west extension that is not related to the extensional collapse

early proterozoic continental tholeiites from western bergslagen central sweden iii geodynamic inferences abstract the 1 88 ga old within plate continental tholeiites belong to the mainly metavolcanic bimodal bergslagen supracrustal series bss in the svecofennian domain of the baltic shield the bss evolved in an intracontinental rift despite only moderate lithospheric extension the tholeiites are the near surface expression of a 10 20 km thick magmatic crustal underplate that probably underlies most of the sw svecofennian domain and caused the svecofennian crust to be three layered thick but yet isostatically stable underplating strengthened the lithosphere sufficiently to control the time of cratonization geodynamic models for the origin of the bss consistent with our interpretations are gravitational collapse of an overthickened continental accreted terrane margin or back arc processes related to a marginal volcanic arc 250 km further south a tentative reconstruction of relative plate motions during svecofennian time indicates predominantly longitudinal motion of the plates in question and complies with the relatively slow latitudinal drift of the shield at the time recent ion microprobe u pb ages of proterozoic zircons in svecofennian metasediments imply about 0 2 ga of pre svecofennian crustal evolution while the nd model ages of the most primitive metatholeiites may be indicative of initial mantle differentiation as early as 2 4 ga bp accompanying protocrust formation

palaeoproterozoic glaciomarine sedimentation in an extensional tectonic setting the honkala formation finland abstract the honkala formation of the honkajarvi group in the kainuu schist belt of northern finland central fennoscandian shield provides evidence of palaeoproterozoic glaciation on the fractured surface of the craton the diamictites and associated rocks of the honkala formation are some 190 m thick in the studied area they consist of a remnant of the glacigenic sequence deposited during an incipient stage of crustal extension the rocks rest unconformably on the late archaean basement and conformably on rift related volcanics the volcanics were erupted during a period of general fragmentation and rifting of the late archaean basement 2 4 2 3 ga ago at this time shallow intracratonic basins and depressions started to form on a continental scale subsequently many of these basins subsided below sea level the lower part of the honkala formation consists of turbiditic non glacial mudstones and sandstones up to 70 m thick glacial advance is indicated by ice rafted deposits forming a succession of some 10 to 50 m deposition took place in a glaciomarine rift basin extending towards the interior of the archaean craton waning glacial influence is indicated by some 70 m of sandstones the uppermost sandstones were influenced by intermittent chemical weathering indicating that these deposits were subjected to subaerial conditions and warmer climate correlation on a global scale is possible during the early palaeoproterozoic the fennoscandian and north american cratons were probably situated close together at a high latitude and both were affected by a major continental glaciation the fennoscandian craton then shifted to lower latitudes whereas the north american cratonic margin recorded further glaciations at high latitudes

preliminary results of a palaeomagnetic and rock magnetic study of the proterozoic tsuomasvarri intrusions northern fennoscandia abstract preliminary palaeomagnetic results are presented for the proterozoic tsuomasvarri central ultramafic intrusion and surrounding older gabbro diorite intrusion in the northeastern fennoscandian shield in the ultramafic intrusion five remanence components reflecting different geological processes were isolated in the alternating field af and thermal demagnetization of 55 samples from ten sites a reversed polarity remanence component c pole plat 26 n plon 245 e dp 5 dm 9 n 32 samples dominates in the ultramafic intrusion the normal polarity pole a plat 44 n plon 232 e dp 4 dm 6 n 9 samples corresponds to an age of 1 88 ga on the apparent polar wander path apwp and is probably a thermochemical remanence acquired during the svecokarelian orogeny or in the late stages of magma cooling pole b plat 35 n plon 158 e dp 9 dm 15 n 5 samples normal polarity corresponds to the 1 75 ga old post orogenic activity in the area and is also seen in results of k ar whole rock isotopic age determinations from the vicinity the normal polarity pole d pole plat 25 s plon 285 e dp 12 dm 19 n 11 samples has a palaeomagnetic age of 2 44 ga which is in contrast with the isotopic age determination of 1 93 ga u pb age on zircon of the gabbro diorite intrusion pole f plat 3 n plon 212 e dp 10 dm 18 n 5 samples indicates an age of 0 5 0 4 ga on the palaeozoic apwp segment of fennoscandia and probably reflects a weak caledonian overprint in northeastern finland in the gabbro diorite intrusion two remanence components were isolated in 46 samples from eight sites the palaeomagnetic age 1 93 ga of the dominant normal polarity component a pole plat 40 n plon 247 e dp 5 dm 8 n 28 samples is consistent with the isotopic age of 1 93 ga u pb determination on zircon the normal polarity component d pole plat 20 s plon 285 e dp 14 dm 23 n 5 samples which was also isolated in the ultramafic intrusion contradicts the u pb isotopic age determination

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

early proterozoic palaeomagnetic data from the pechenga zone north west russia and their bearing on early proterozoic palaeogeography summary an early proterozoic palaeomagnetic signature c 2125 ma verified by a positive conglomerate test is recorded in the kuetsyarvi formation pechenga group north west russia but the majority of the palaeomagnetic directions observed in the pechenga group lithologies reflect a low grade remagnetization event probably linked with the late precambrian baikalian orogeny which affected north west russia and northern norway secondary pyrrhotite is the dominant remanence carrier in the uppermost formations of the pechenga group palaeomagnetic poles from the kuetsyarvi formation differ somewhat or partially overlap with coeval palaeomagnetic poles from other tectonomagmatic provinces in northern fennoscandia but it is premature at this stage to speculate on intraplate movements during the early proterozoic besides the kuetsyarvi formation probably developed during an early phase of intracontinental rifting along the northern margin of fennoscandia similar to the present day east african rift hence younger intercontinental rifting possible seafloor spreading and subsequent convergence would remain undetected by our palaeomagnetic data palaeolatitude estimates from the kuetsyarvi formation suggest that the pechenga region was located in latitudes of around 20 to 30 during the 2100 2200ma interval these low latitude estimates are supported by the sedimentary record in the pechenga region which is characterized by red beds and evaporites comparison of fennoscandian palaeomagnetic poles with coeval poles from the slave and superior cratons laurentia questions previously publicized supercontinental configurations a close relationship between fennoscandia and early proterozoic laurentian provinces is conceivable from palaeomagnetic data but given the lack of longitudinal control as well as the choice of hemisphere such postulates are tentative at best on purely palaeomagnetic grounds

absolute u pb and relative age determinations of intrusive rocks in the ragunda rapakivi complex central sweden abstract the ragunda complex ca 550 km2 is one of the two major rapakivi occurrences in central sweden and forms together with seven smaller complexes the youngest group 1 53 1 47 ga of rapakivi complexes in the fennoscandian shield in contrast to most of the classic finnish rapakivi plutons the ragunda complex consists of large amounts of mafic rocks and a substantial volume of syenite and quartz syenite at the present erosion level the ragunda complex includes ca 30 gabbro 5 syenite and quartz syenite and 65 amphibole biotite and biotite granites minor amounts of hybrid rocks and a large number of acid and basic dykes are found as well the complex consists of three massifs with distinctive aeromagnetic anomalies the western massif is a centred complex in which magmatic activity began in the west and moved towards the east forming a linear sequence of approximately circular gabbro syenite and granite intrusions the last intrusion a amphibole biotite granite gave a u pb zircon age of 1514 5 ma the central massif is a large centred complex without any obvious linear intrusion sequence and consists of several consecutively emplaced plutons including a late granite with a u pb zircon age of 1505 12 ma the northeastern massif is a small centred complex consisting of gabbro closely followed by granite magma mingling structures are observed throughout the complex most commonly in contact zones between gabbro and granite the generation of the ragunda complex and the small complexes in central sweden cannot be explained by a lithospheric plate moving over a stationary hot spot deep fractures formed in response to an extensional regime probably controlled the emplacement of the complexes this is supported by the large number of subparallel mafic and acid dykes associated with them

global plate motions relative to the hot spots 64 to 56 ma we examined the early tertiary global plate velocities with respect to the hot spots and compared these velocities both to those at present and to early tertiary velocities inferred by assuming that no net torque is exerted on the lithosphere as a whole in our reconstruction the velocities of the pacific and african plates were inferred directly from their hot spot tracks and the velocities of other plates with respect to the hot spots were calculated from the motion of the pacific or african plate and by relative motions estimated from seafloor spreading data the relative motion between assumed separate north pacific and south pacific chatham rise plates was estimated by assuming the hot spots in the atlantic ocean basin have been fixed with respect to those in the pacific ocean basin thereby avoiding the use of a poorly defined relative motion circuit we found that the characteristics of early tertiary plate motions with respect to the hot spots resemble those at present the root mean square velocity of every major continental plate with respect to the hot spots during the early tertiary exceeds its present velocity but with the exception of the indian plate is less than the root mean square velocity of every early tertiary oceanic plate equatorial lithosphere moved faster than polar lithosphere during the early tertiary but the difference is less than at present over the interval 64 to 56 ma the no net torque absolute velocities differ from the assumed fixed hot spot velocities by only 0 04 ma which is insignificant thus two dissimilar approaches fixed hot spots and no net torque of the lithosphere when applied to identical early tertiary relative plate velocities yield similar absolute plate velocities we attribute the differences between these two reference frames found in prior investigations to relative plate velocities different than we assume here

contiguous laurentia and baltica before the grenvillian sveconorwegian orogeny close geochronological correspondence of mesoproterozoic extension related magmatism and comparable tectonic and geochronological evolution of the grenville and svecononvegian orogens suggests that the laurentian and baltic shields could have been joined during the mesoproterozoic and did not separate until after the grenvillian sveconorwegian orogeny the combined grenville svecononvegian orogen is here interpreted to have formed during the collision of coherent laurentia baltica with an unknown craton which is in contrast to earlier explanations that favour a late mesoproterozoic rotation between laurentia and baltica followed by collision of these two cratons with each other

rock magnetic and paleomagnetic study of the keurusselka impact structure central finland abstract there are 31 proven impact structures in fennoscandia one of the most densely crater populated areas of the earth the recently discovered keurusselka impact structure 62 08 n 24 37 e is located within the central finland granitoid complex which formed 1890 1860 ma ago during the svecofennian orogeny it is a deeply eroded complex crater that yields in situ shatter cones with evidence of shock metamorphism e g planar deformation features in quartz new petrophysical and rock magnetic results of shocked and unshocked target rocks of various lithologies combined with paleomagnetic studies are presented the suggested central uplift with shatter cones is characterized by increased magnetization and susceptibility the presence of magnetite and pyrrhotite was observed as carriers for the remanent magnetization four different remanent magnetization directions were isolated 1 a characteristic svecofennian target rock component a with a mean direction of d 334 8 i 45 6 95 14 9 yielding a pole plat 51 1 plon 241 9 a95 15 1 2 component b d 42 4 i 64 1 95 8 4 yielding a pole plat 61 0 plon 129 1 a95 10 6 3 component c d 159 5 i 65 4 95 10 7 yielding a pole plat 21 0 plon 39 3 a95 15 6 and 4 component e d 275 5 i 62 0 95 14 4 yielding a pole plat 39 7 plon 314 3 a95 19 7 components c and e are considered much younger possibly neoproterozoic overprints compared with the components a and b the pole of component b corresponds with the 1120 ma pole of salla diabase dyke and is in agreement with the 40ar 39ar age of 1140 ma from a pseudotachylitic breccia vein in a central part of the structure therefore component b could be related to the impact and thus represent the impact age

two dimensional modelling and interpretation of seismic wide angle data from the western gulf of bothnia in late 1989 wide angle reflection and refraction profiles were shot in the baltic sea the bothnian sea and the gulf of bothnia in order to study the lithospheric structure of archaean and proter

geophysical interpretation of the crustal and upper mantle structure in the wiborg rapakivi granite area southeastern finland abstract the rapakivis in southeastern finland and russian karelia cover a total area of more than 20 000 km 2 and comprise the proterozoic 1 6 1 3 ga anorogenic wiborg ahvenisto suomenniemi and onas batholiths and associated mafic rocks a model of the crustal and upper mantle structure in this area based on gravity aeromagnetic deep seismic sounding and petrophysical data is presented the average density 2625 kg m 3 of the batholiths is lower than that of the surrounding svecofennian rocks by about 80 kg m 3 the density histogram of the rapakivi granites is divided into two normally distributed components with mean densities of 2625 kg m 3 major and 2690 kg m 3 minor respectively there are related local gravity anomalies the susceptibility histogram of the rapakivi granites is decomposed into five log normally distributed components the two major components have mean susceptibilities of 165 and 1305 10 6 si units the former component 40 is virtually paramagnetic and the latter 60 is weakly ferromagnetic the two components with mean susceptibilities of 6000 and 13 5000 10 6 si units constitute only 3 of all the samples associated gabbro anorthosites also have high susceptibilities 1450 30 370 10 6 si units these ferrimagnetic rocks cause noticeable local aeromagnetic anomalies an extensive bouguer anomaly low is associated with the batholiths a pronounced regional high surrounds this low the range of values is nearly 60 mgal the anomaly pattern is primarily the result of superposition of the gravity low which is due to the relatively low density upper and middle crust onto the broader gravity high that is due to the uplifted moho according to the published deep seismic sounding results the crust is 41 km thick in the rapakivi region thus it is 6 20 km thinner than to the west north and south of the rapakivi batholiths the refraction model also implies a 6 km thick high velocity body at a depth of 10 km this body has a high v p v s ratio and is interpreted as gabbro anorthosite the magnetic anomaly values in the rapakivi area are slightly higher than those in the surrounding schist areas but lower than the values in the metavolcanic belt to the west within the wiborg batholith there is a zone of positive regional magnetic anomalies with a separate well defined magnetic high of about 400 nt in the western part this anomaly is interpreted as a pipe like magnetic body with an abnormally strong magnetization the upper surface of the body lies at depths between 12 and 15 km the body extends from the upper to the lower crust the model derived from geophysical data supports a proterozoic mantle upwelling which provided excess heat to cause uplift and partial melting of both the upper mantle and the lower crust partial melting of the upper mantle produced intrusions of gabbro anorthosites and diabases while extensive partial melting of the lower crust produced rapakivi magmas and caused their emplacement into the upper crust

models of crustal anatexis in volcanic rifts applications to southern finland and the oslo graben southeast norway summary crustal anatexis in volcanic rifts is discussed in terms of a 1 d numerical model incorporating lithospheric extension emplacement of hot mafic magma at crustal levels various crustal compositions and pre rift conditions only the volume of melt generated by anatexis is considered crustal melt segregation transport and emplacement at higher levels are not included we find that under a broad range of conditions influx of mafic magma to the crust is required for substantial anatexis to take place and once this stops silicic magmatism dies out rapidly in many rifts the observed change in type of magmatism from predominantly mafic to silicic during rifting is probably more due to a diminishing potential for mafic magmas to ascend though the crust than to an actual halt in mafic melt generation at depth this has important implications where melt production rates are used to analyse the formation of volcanic rifts and margins when influx of mafic magmas takes place over a finite period of time they may be focused in the depth region where the potential for anatexis is largest and in some cases more anatexis can occur than by instantaneous emplacement of the entire mafic body our results support the idea that the 1 6 gyr old rapakivi granites in southern finland were formed by anatexis of an intermediate to felsic svecofennian crust provided a mantle thermal anomaly of 200 250 c existed beneath finland the seismically observed 10 km thick magmatic underplated body could have produced the exposed silicic rocks which are on average 2 3 km thick in the oslo graben in southeast norway where the thickness of magmatic underplating is also about 10 km the amount of rock produced by anatexis is poorly constrained but appears to be between 1 and 2 km provided the pre rift crustal thickness in the graben was 35 40 km or more and heat was transported to crustal levels more eyciently than is feasible by conventional stretching models the model predicts melt volumes in agreement with these estimates we propose that secondary convection within the narrow graben could have contributed to the high heat transport eyciency finally a conspicuous feature of the southern finland magmatic region as well as the oslo graben is the large discrepancy between stretching factors derived from analysing dyke intrusions and faulting finland b 1 001 oslo graben b 1 05 and by using variations in crustal thickness finland b 2 oslo graben b 1 7 2 a possible explanation for this diverence may be that when large volumes of mafic rocks cause crustal anatexis the partially molten crust becomes very weak and acts as a barrier for stress propagation evectively isolating the upper crust from further deformations this could add to the thermal weakening of the ductile lower crust due to the influx of hot mafic magmas this hypothesis could be tested by investigating a large number of magmatic rifts combined with development of new rheological models of the lithosphere including the evects of underplating and anatexis

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

palaeomagnetism of vendian early cambrian sedimentary rocks from e finnmark norway abstract a palaeomagnetic study has been made on vendian early cambrian rocks of the tanafjord and vestertana groups from the varanger peninsula northernmost norway magnetic components of probably primary origin were isolated at two sites apparently unaffected by the finnmarkian scandian events of the caledonian orogeny sites situated in the caledonian gaissa nappe or close to its front were remagnetized and comparison with a tentative ordovician to devonian apparent polar wander path indicates that the remagnetization occurred during these periods

morphosedimentary features and recent depositional architectural model of the cantabrian continental margin abstract multibeam bathymetry high sleeve airguns and very high resolution parametric system topas seismic records were used to define the morphosedimentary features and investigate the depositional architecture of the cantabrian continental margin the outer shelf down to 180 245 m water depth displays an intensively eroded seafloor surface that truncates consolidated ancient folded and fractured deposits recent deposits are only locally present as lowstand shelf margin deposits and a transparent drape with bedforms the continental slope is affected by sedimentary processes that have combined to create the morphosedimentary features seen today the upper down to 2000 m water depth and lower down to 3700 4600 m water depth slopes are mostly subject to different types of slope failures such as slides mass transport deposits a mix of slumping and mass flows and turbidity currents the upper slope is also subject to the action of bottom currents the mediterranean water mw that interact with the le danois bank favouring the reworking of the sediment and the sculpting of a contourite system the continental rise is a bypass region of debris flows and turbidity currents where a complex channel lobe transition zone cltz of the cap ferret fan develops the recent architecture depositional model is complex and results from the remaining structural template and the great variability of interconnected sedimentary systems and processes this margin can be considered as starved due to the great sediment evacuation over a relatively steep entire depositional profile sediment is eroded mostly from the cantabrian and also the pyrenees mountains source and transported by small stream river mountains to the sea it bypasses the continental shelf and when sediment arrives at the slope it is transported through a major submarine drainage system large submarine valleys and mass movement processes down to the continental rise and adjacent biscay abyssal plain sink factors controlling this architecture are tectonism and sediment source dispersal which are closely interrelated whereas sea level changes and oceanography have played a minor role on a long term scale

peat or no peat why do the rajang and mahakam deltas differ abstract coastal and deltaic holocene peat accumulations around the equatorial island of borneo southeast asia have served as models for economic coal bearing sequences in the stratigraphic record although climatic conditions vegetational communities and sedimentary regimes are comparable peat accumulations are not found on both the western and eastern sides of the island the rajang river delta and coastal plain sarawak east malaysia are covered in areally extensive thick peat deposits that have attained at least a thickness of 13 m in ombrogenous peat domes marudi baram river peat swamp biomass began to accumulate over pleistocene podzols when sea level stabilized 7 5 ka and delta progradation was initiated the mahakam river delta and coastal plain east kalimantan indonesia also began progradation at this time but there is no evidence in any part of the coastal region for peat accumulation rather poorly developed organic rich gleysols occur throughout the delta plain both the rajang river and mahakam river deltas are tidally influenced fine grained systems with a sediment provenance in the central massif sediment transported through the rajang river delta differs in that as much as 60 of the clay minerals deposited in the system are mixed layer i s and expandable k e clays that act to restrict pore water flow in the tidal and overbank deposits that comprise the delta plain these result in the development of an aquiclude above which paludal conditions develop promoting accumulation of organic matter in contrast there is a low proportion of mixed layer and expandable clays transported in the mahakam river system this precludes the development of a stilted water table within the delta allowing for organic matter recycling without peat accumulation the presence of a high proportion of expandable clay minerals on the western side of borneo is a reflection of the weathering and eroding source rocks on this side of the central borneo massif

palaeomagnetism and the siljan impact structure central sweden summary the siljan impact structure diameter c 52 km located in central sweden is considered the largest meteorite crater in western europe the time of impact has been estimated to be 368 ma from ar ar dating of an impact melt a palaeomagnetic study of rocks from inside and outside the impact structure has been carried out with the aim of identifying natural remanent magnetizations nrms related to the impact event primary magnetizations are partly preserved within the impact area however the directions of these original magnetizations are disturbed due to tectonic movements related to the impact three different directions of magnetizations not recognized elsewhere were identified within the central part of the structure these three magnetizations are defined in order of increasing coercivity hc and blocking temperature tb the origin of a high hc chemical remanent magnetization crm c of late palaeozoic time carried by haematite can be interpreted as due to oxidation caused by circulating meteoric water in cracks and fractures caused by the impact another component b identified in a lower temperature range 40d 580dc is probably carried by magnetite in pseudotachylite this magnetization is considered as a thermal remanent magnetization while in other rocks its pre tectonic character and occurrence also in the low coercivity range implies a shock remanent origin the palaeomagnetic pole for this remanence corresponds to a silurian devonian age which gives an age of impact which is not very diffirent from that estimated by radiometric data a third low hc and tb component a is interpreted to be a crm carried by slowly growing goethite the direction of this crm implies it to be a recent post impact magnetization

the quasi rigid premise in precambrian tectonics abstract the proposition that movements between multiple continental fragments driven by plate tectonics have taken place during the earlier eons of geological time can be simply tested a random distribution will be derived when the collective palaeomagnetic poles are rotated into any quasi rigid reconstruction this elementary test surmounts the most serious limitation of precambrian palaeomagnetic data namely that the ages of magnetisation are mostly poorly known in the event the late archaean and proterozoic poles 2850 590 ma produce a non random distribution with a single peak which is only compatible with a quasi rigid continental configuration occupying a preferential position on the globe this solution is not favoured because uncertainties in definition of the poles contribute to their scatter and bias the solution towards the random one hence confirmation of the quasi rigid premise is derived in a debased form but is not suppressed by the deficiencies of the database this palaeomagnetic constraint explains the distinctive isotopic geochemical and lithofacies signatures of the proterozoic eon and is independently supported by the trend and long temporal continuity of crustal lineaments it does not preclude the opening of ocean basins between the shields but it does predict that development of these basins was limited by the influence of controlling mantle systems this constrained the continental crust into a form which was essentially rigid on the gross scale resolvable by palaeomagnetism

filtering spatial error from dems implications for morphological change estimation abstract scour and fill estimation from digital elevation model dem subtraction or differencing is an increasingly common technique in morphological and sediment transport investigations the technique is commonly used to estimate scour and fill volumes and to produce scour and fill maps that provide process based information to geomorphologists accounting for sources of uncertainty within the dem is of critical importance dem error is spatially variable and has a tendency to be greater at breaks of slope such as bar and bank edges in the past however this has been achieved using a uniform error metric across the dem resulting in over conservative estimates of error in turn this has led to over conservative scour and fill volumes and incorrect process interpretation this paper applies a new approach that permits assessment of spatially distributed error across a dem the method is tested on a sequence of field surveys of the gravel bed river nent cumbria uk the results demonstrate some dramatic differences application of conventional techniques that account for mean error across a dem led to a 15 and 31 underestimation in scour and fill volumes respectively between july and october 1998 whilst for the october 1998 june 1999 subtraction 31 and 13 of scour and fill were underestimated respectively use of a uniform error across a surface captures less change in comparison to a spatially distributed approach furthermore the changes captured using a uniform error are biased toward areas of the channel that have more local topographic variability such as bar and bank edges in contrast the use of a spatially distributed approach provides information on change from flatter surfaces such as bar tops that would otherwise be missed this study also demonstrates that estimates of morphological change can be misleading in the absence of an error filter where the raw survey data is available it is recommended that sediment budgeting studies take account of the spatial variability of error in each dem involved in the subtraction

human induced stream channel abandonment capture and filling of floodplain channels within the atchafalaya river basin louisiana article i nfo the atchafalaya river basin is a distributary system of the mississippi river containing the largest riparian area in the lower mississippi river valley and the largest remaining forested bottomland in north america reductions in the area of open water in the atchafalaya have been occurring over the last 100 years and many historical waterways are increasingly filled by sediment this study examines two cases of swamp channels b85 m 3 s that are filling and becoming unnavigable as a result of high sediment loads and slow water velocities the water velocities in natural bayous are further reduced because of flow capture by chan nels constructed for access bathymetry flow suspended sediment deposited bottom material isotopes and photointerpretation were used to characterize the channel fi ll on average waterflowing through these two channels lost 23 of the suspended sediment load in the studied reaches along one of the studied reaches two constructed access channels diverted significant flow out of the primary channel and into the adjacent swamp immediately downstream of each of the two access channels the cross sectional area of the studied channel was reduced isotopic analyses of bottom material cores indicate that bed filling has been rapid and occurred after detectable levels of cesium 137 were no longer being deposited interpreta tion of aerial photography indicates that water is bypassing the primary channels in favor of the more hy draulically efficient access channels resulting in low or no velocity flow conditions in the primary channel these swamp channel conditions are typical in the atchafalaya river basin where relict large chan nel dimensions result in flow velocities that are normally too low to carry fine grained sediment constructed channels increase the rate of natural channel avulsion and abandonment as a result of flow capture

palaeomagnetic and rock magnetic investigations of the sipoo subjotnian quartz porphyry and diabase dykes southern fennoscandia abstract the sipoo quartz porphyry dyke swarm u pb zircon age 1 63 ga in southern finland carries an intermediate to high coercivity remanent magnetization a n d 22 2 i 1 8 k 34 2 95 13 3 n 5 dykes which is regarded as primary the sipoo diabase dykes emplaced slightly before the granite porphyries yield a highly scattered antiparallel direction a r d 198 0 i 6 5 k 6 2 95 40 1 n 4 dykes isolated in intermediate to high coercivities based on positive contact test results remanence a r is primary in the diabases the best preserved diabase dyke as determined by petrographic and bulk magnetization properties carries both the a r magnetization and the a n magnetization as a secondary component in some samples the most altered diabase dyke is almost completely overprinted by a n the overprinting has taken place as a result of hydrothermal alteration and formation of new magnetite during the emplacement of quartz porphyry dykes and rapakivi granite the quartz porphyries and diabases carry mixed sd and md magnetic grain sizes the large md grains being responsible for a relatively large present earth s field pef magnetization in addition a high coercivity magnetization b d 23 9 i 52 2 k 215 8 95 6 3 n 4 dykes presumably a chemical overprint has been isolated in the quartz porphyries the b magnetization is interpreted to be either an intermediate direction between a n and pef with no geological significance or to represent a separate reactivation of the crust at about 1 32 ga ago

the gyroscopic earth and its role in supercontinent and metallogenic cycles abstract compilation and analysis of the paleomagnetically constrained plate tectonic reconstructions for the last 2 5 ga of earth s history reveal that continental and oceanic hemispheres are principal and stable first order divisions on the earth with cyclical breakup and reassembly of the continental crust into supercontinents although the breakup of the supercontinent rodinia at 0 75 ga marked important reorganization of the plate tectonic pattern the movements of large continental masses both before and after the breakup of rodinia show remarkable synchronism albeit in significantly rearranged combinations the patterns with disassembled continents coincided with periods when substantially more than 50 of the continental masses occurred either in the northern during palaeoproterozoic and cenozoic times or southern during neoproterozoic to middle palaeozoic times hemispheres whereas the reassembled supercontinents were always symmetrically centred near the equator it is proposed that such regularity might be governed by a convection driven move of the continental fragments towards a pole after the breakup of the supercontinent followed by gyroscopic rebalancing or shift of all earth s solid shells e g entire mantle lithosphere towards the equator relative to the more stable oriented magnetic currents in the liquid core this process is somewhat similar to the true polar wander concept but it takes into account the spinning forces of the earth mantle convection is considered as an important force constantly driving the plates in the oceanic hemisphere and keeping the continental hemisphere intact the periods of gyroscopic rebalancing correspond to the reassembly of the supercontinents at 2 7 2 5 ga kenorland 2 0 0 75 ga columbia and its modification into rodinia and 0 32 0 18 ga pangaea the main reassembly mechanism in addition to rifting spreading and collision is large scale strike slip translation of not only relatively small lithotectonic terranes but also of major cratons these cycles govern changes from the dominantly extension to collision and plume related mineral deposit types in the internal orogens in the continental hemisphere whereas subduction related to collision related mineral deposit types remain persistent through the metallogenic cycles at the oceanic continental hemisphere transition zone just migrating oceanward in time

palaeomagnetic data bearing on the age of the ytteroy dyke central norway abstract the ytteroy dyke has a near isotropic fabric and clearly post dates caledonian regional deformation it has a two component magnetization structure with a low blocking temperature component d 016 i 73 95 9 removed below 300 c the high blocking temperature component d 216 i 42 95 8 could be primary in origin the palaeomagnetically obtained age accords well with an rb sr biotite age of 256 10 ma and it could be argued that intrusion of the ytteroy dyke was related to extensive permian rifting and faulting within the more trondelag fault zone tentatively we ascribe the previously claimed late devonian age k ar and 40 ar 39 ar biotite ages for the ytteroy dyke as resulting from the presence of excess ar or alternatively k loss induced by a later hydrothermal flux

uplift and cooling magnetisation record in the bamble and telemark terranes sveconorwegian orogenic belt se norway and the grenville sveconorwegian loop article i nfo the 1100 ma sveconorwegian orogenic belt comprises allochthonous terranes juxtaposed by major fault zones and emplaced against and onto the south western margin of the fennoscandian shield to resolve the magnetic signature acquired during post orogenic uplift and cooling and evaluate wider correlations with the contemporaneous grenville belt of north america this study reports a regional palaeomagnetic study on a range of rock types from sectors of the medium high metamorphic grade bamble terrane 48 sites and 390 cores and the adjoining medium low grade telemark terrane 33 and 240 cores juxtaposed by an orogen parallel porsgrunn kristiansand fault zone with major vertical displacement magnetite and ilmeno hematites are magnetic carriers with the latter more significant in the higher metamorphic grades magnetic intensities are stronger in the higher grade terrane presumably due to the growth of metamorphic ferromagnets but are an order lower than values predicted for the lower continental crust and indicate that an additional mechanism is responsible for high magnetisations in deep crust anisotropy of magnetic susceptibility ams largely reflects the ne sw tectonic grain of the last stage of sveconorwegian ductile deformation the magnetisation record is filtered by excluding magnetisations possibly acquired during regional mesozoic dyke emplacement development of the permo carboniferous oslo rift and late proterozoic magmatism the remaining record is a dual polarity signature summarised by mean poles at 31 9 n 50 9 e n 191 components in the bamble terrane and at 34 2 n 58 9 e n 151 components in the telemark terrane however these means are non fisherian and embrace arcuate distributions of magnetic components acquired during protracted exhumation cooling of the orogen with the best defined parts

did the ebro basin connect to the mediterranean before the messinian salinity crisis the connection of the ebro basin via the ebro river to the mediterranean sea is supposed to have played a major role in the rejuvenation of relief of northern spain and especially of the south pyrenees from the neogene by lowering the initial base level of the ebro internal drainage area down to sea level however the timing and causes of this connection are still debated a fundamental issue is whether the ebro basin became connected to the mediterranean before or after the messinian salinity crisis morphological analysis and numerical modeling of landscape evolution show that this connection did not exist before the messinian salinity crisis but is effective from the pliocene because of progressive regressive erosion

structure and composition of the precambrian crust along the polar profile in the northern baltic shield abstract along the 500 km long polar profile the continental crust of the northern baltic shield consists of several archaean and early proterozoic crustal segments in the south in the karelian province the upper crust consists of a 3 1 2 6 ga old archaean substratum with a 0 6 km thick low to medium grade volcanosedimentary cover here shallow basin facies rocks of the central lapland complex have been thrust northwards onto the karasjok kittila greenstone belt the repeatedly folded cover rocks form linear troughs between domes and culminations of granitoids the 47 48 km thick crust of the karelian province is of a shield type it is characterised by a low velocity layer interpreted as a horizon of decollement below the supracrustal and granitoid rocks in the lapland granulite belt early proterozoic high grade metapelites and metapsammites with minor metaigneous intercalations have been thrust to the southwest they are separated from the underlying karelian province by the tectonic mixture zone of the tanaelv belt in the interpretations two alternative models are given for the shape of the lapland granulite belt it is either a synformal nappe or a ne dipping body pinching out at depth and overthrust from the north by the archaean inari terrain both the lapland granulite belt and the inari terrain have a 42 44 km thick crust with a relatively thin lower crust marked by lower seismic velocities the early proterozoic polmak pasvik pechenga belt separates the archaean inari terrain from the archaean sorvaranger terrain in the north this belt is interpreted as a suture zone which continues to depth it may either dip steeply towards irregularities in the moho or form a sw dipping low angle plane below the lapland granulite belt the northernmost part of the baltic shield is concealed beneath a 0 7 km thick very low grade metamorphic late proterozoic cover this cover is divided into two parts by the trollfjord komagelv strike slip fault here the crust is again of a normal shield type with a thickness of 42 46 km

1 9 1 8 ga old strike slip megashears in the baltic shield and their plate tectonic implications abstract the tectonic evolution of the ca 2 0 1 75 ga old svecokarelian fold belt is reviewed and evidence is presented for large scale intraplate strike slip movements along ductile megashears after the formation of the kola collision suture and the neighbouring granulite tanaelv thrust belt around 1 9 ga ago dextral shearing was initiated along n s trending megashears subsequent anticlockwise rotation of the initially nne ssw oriented principal compressive stress caused dextral shearing along a nw se trending megashear and reversal in the sense of shearing in the n s trending ones further anticlockwise stress rotation to a total of about 120 brought an end to sinistral shearing along the n s megashears around 1 8 ga ago and caused reversal to sinistral slip along the nw se megashear both the older 1 9 1 85 ga and younger 1 84 1 8 ga parts of this evolution are recorded within the karelian province and its southwestern margin where consolidation of the lithosphere took place shortly after 1 9 ga ago in the svecofennian province where crustal accretion did not start until around 1 9 ga ago the older movements may have caused synaccretional crustal folding but with increasing consolidation the deformation was concentrated along megashears although it is still not possible to interrelate the function of active subduction zones and intraplate megashears the evolution traced so far provides support for plate tectonic interpretations of the early proterozoic geodynamics of the baltic shield

palaeomagnetism of the tudor gabbro ontario evidence for divergence between grenvillia and interior laurentia summary after thermal and alternating field af cleaning the characteristic high blocking temperature a component of natural remanent magnetization nrm of the tudor gabbro of southern ontario has a mean direction d 326 i 46 k 132 95 4 8 n 8 sites the corresponding palaeopole 133 e 12 n dp 4 dm 6 confirms the palaeopole 137 e 17 n 95 8 4 reported earlier by palmer carmichael based on af cleaning only the a nrm has unblocking temperatures 515 525 c which exceed the estimated 500 c peak temperature reached locally during 1050 ma grenvillian regional metamorphism the a nrm therefore predates metamorphism and is probably a primary thermoremanence trm the age of the tudor nrm has previously been taken to be about 675 ma but recent 40ar 39ar dating by baksi has shown that this is the time of post metamorphic cooling to 200 250 c hornblendes record initial cooling of the intrusion to 590 20 c at 1110 ma and this is the best estimate of the age of the a remanence successful thellier type palaeointensity determinations on 11 tudor samples confirm that the a nrm is a trm and indicate a palaeofield at this time of 18 27 t about 50 70 per cent of the present field intensity at 27 magnetic latitude the anomalous tudor a palaeopole which lies well to the west of both 1000 800 ma grenvillian palaeopoles and 1100 1050 ma poles from interior laurentia is interpreted as recording divergence between grenvillia and interior laurentia just before the grenvillian orogeny rather than a post metamorphic extension of the apparent polar wander path as previously assumed

secondary fe mn oxides in minerals heavily damaged by α recoil possible implications for palaeomagnetism sub micron fe mn oxides in columbite tantalite minerals are bound to metamict domains in the host these nano oxides are secondary minerals as the metamict zones formed through accumulation of damages from recoil each of which in a small volume destroys the crystal lattice of the u and th bearing columbite tantalite host transmission electron microscope investigations demonstrate that the oxides fall in the compositional range of magnetite jacobsite iwakiite fe mn 3o4 and of hematite type minerals along the fe2o3 mn2o3 join i e minerals that are known to be major carriers of magnetic remanence measured magnetic properties of isolated columbite tantalite crystals demonstrate in some samples magnetic remanence which is bound to sub micron pseudosingle domain crystals rather than to the host thus the magnetic remanence is not related to the formation of columbite tantalite but to the crystallization of the nano oxides which occur in metamict domains of the host and therefore are secondary depending on the content and distribution of u and th the nano oxides may form diachronously within an individual and among several host crystals magnetic secondary nano oxides are not restricted to columbite tantalite minerals but may occur in other minerals where high contents of fe and mn are associated with high contents of u and th rocks that show the same spatial distribution for u and fe as for instance some red sandstones may display magnetic properties that are controlled by recoil induced growth of secondary fe oxides

palaeomagnetism of proterozoic dykes from bornholm denmark summary the island of bornholm denmark is a structural horst situated at a kink on the margin of and linking two major fault zones the sorgenfrei tornquist zone stz and the tornquist teisseyre zone ttz the fault zones of which separate the downfaulted danish polish basin from the baltic shield thirteen dolerite dykes out of a swarm of more than 200 all belonging to the supposed precambrian n s to ne sw striking dyke swarm on bornholm have been sampled and palaeomagnetically investigated the dykes vary in thickness between 0 2 and 60m and are intruded in precambrian gneisses and granites of gothian and or subjotnian 1 650 1280 ma age principal component analysis of detailed af and some thermal demagnetization experiments reveal three groups of stable direction clusterings from the dykes investigated the stable remanent magnetization of the major dyke keldse2 dyke and the baked gneiss contact corresponds to a palaeomagnetic mean pole group a comparing it with the apw curve for fennoscandia an initial cooling age for this dyke of around 1360 ma subjotnian age is suggested the result is confirmed by dual polarities and a positive baked contact test nine other dykes show a magnetization corresponding to a younger group of poles group b which cluster on the apw path around 1200ma jotnian age this result is also confirmed by a positive contact test two poles group c fall on the apw path around 950ma sveconorwegian age remaining directions display a distribution of poles on the sphere which suggest the presence of either late proterozoic cambrian overprints or injections of some younger dykes based upon a total of 16 bornholm dykes hitherto studied the polarity frequency ratio nir is 719 the remanent magnetizations and equivalent palaeomagnetic poles found in the bornholm dyke swarm thus appear to indicate at least three different periods of igneous activity with dyke injections in early proterozoic time for this otherwise homogeneously looking group of dykes from the margin of the baltic shield

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

global and continental scale glaciations on the precambrian earth abstract earth s climate during the proterozoic eon was marked by major glacial events with evidence for large continental ice sheets on many cratons and with sedimentological data indicating that glaciers had extended to sea level this paper emphasizes the sedimentological and sequence stratigraphic responses to glaciations and evaluates the major driving forces of glaciations during the precambrian first and second order sequences are recognized related to continental scale fragmentation and formation of marine rift basins wherein sedimentary rocks indicate glacial influences and pronounced tectonic climatic linkages coarse syn rift deposits are typically characterized by mass flow diamictites and conglomerates it is important to undertake sedimentological facies and sequence stratigraphic analysis of these syn rift and capping passive margin sequences as well as of slope turbidite deposits which formed if enhanced subsidence of the basins was occurring more generally latitude and syn rift tectonic uplift can cause formation of glacial ice and enhance distinctive glacial influences on extensional basin sedimentation thus supporting a causal relationship between thermal subsidence and the formation of glacier ice on inland areas during the precambrian however it is suggested that long lived marine terminated glaciers also situated at low paleolatitudes were related to an extensional tectonic setting in such settings glacial deposits associated with sedimentary sequences of distinctively different origin e g carbonate and chemically mature siliciclastic sequences can well be used to detect the prominent sequence boundaries to verify depositional systems tracts internal sediment stacking patterns in sequences are indicative of dynamic processes along glaciated continental margins and without always having the need for global synchroneity in glacially influenced rift basins and continental margins it is important to recognize the sequence boundaries of significant subaerial unconformities and their correlative conformities a sequence boundary is a chronostratigraphically significant surface always produced as a consequence of a change in relative sea level these can then be well related to initiation and decay of glaciations however on land glacial deposits in a very few cases are prevented from later erosion attenuation of continental crust during rifting and breakup of the continent raises relative sea level and also many of the shallow intra cratonic basins subsided below sea level in favourable conditions being affected by major continental glaciations paleoproterozoic and neoproterozoic glacial deposits are known in north and south america south africa india western australia and fennoscandia against this background continental scale to global glaciations in the precambrian appear to be possible however views on the causes and timing of these glaciations as well as on planetary extent of ice cover are still contradictory there is a need to continue detailed sedimentological studies of pre glacial and post glacial deposits as well as to interpret syn glacial lithofacies for their inferred transportation and depositional processes pre glacial deposits especially should provide a new target to help us understand the processes that initiated these precambrian glaciations the sequence stratigraphic approach with understanding of the stacking pattern of depositional systems could prevent oversimplification and use of just single events to explain the complexity of evolution of glacially influenced precambrian continental margin sediments

receiver function analysis of the crust and upper mantle in fennoscandia isostatic implications abstract the mountains across southern norway and other margins of the north atlantic ocean appear conspicuously high in the absence of recent convergent tectonics we investigate this phenomenon with receiver functions calculated for seismometers deployed across southern fennoscandia these are used to constrain the structure and seismic properties of the lithosphere and primarily to measure the thickness and infer the bulk composition of the crust such parameters are key to understanding crustal isostasy and assessing its role or lack thereof in supporting the observed elevations our study focuses on the southern scandes mountain range that has an average elevation 1 0 km above mean sea level the crust mantle boundary moho is ubiquitously imaged and we occasionally observe structures that may represent the base of the continental lithosphere or other thermal chemical or viscous boundaries in the upper mantle the moho resides at 25 30 km depth below mean sea level in southeastern coastal norway and parts of denmark 35 45 km across the southern scandes and 50 60 km near the norwegian swedish border that section of thickest crust coincides with much of the transscandinavian igneous belt and often exhibits a diffuse conversion at the moho which probably results from the presence of a high wave speed mafic lower crust across inner fennoscandia a zone of thinned crust

sediment flux to basins causes controls and consequences there is an increasing trend in the earth sciences towards the integration of many subdisciplines the sedimendatry basin is a fundamental focal point of many studies which as a consequence often neglects the complimentary drainage basin or catchment sedimentary basins provide a record of earth history reflecting the geographical lithological oceanographic and ecological development through the rock record drainage basins in comparison record ephemeral landscape evolution where topography is eroded and provides the flux of sediment to the basin the basin fill reflects the sediment flux from the hinterland and provides evidence of the dynamic geomorphic processes in context the drainage system and sedimentary basin can be regarded as a production line with sedimentary record giving valuable insight into long term landscape evolution and geomorphological processes illuminating the evolution of sedimentary basins this volume assesses the current position of understanding sediment supply to basins with the integration of the many sub disciplines in the earth sciences it documents a mix of hinterland and sedimentary basin studies with a gradation from orogenic belts to the deep marine the authors represent a wide spectrum of earth scientist with leaders in the science providing review papers and new directive papers in their field of specialization

milankovitch scale multi proxy records from fluvial sediments of the last 2 6 ma pannonian basin hungary abstract the continuous cores of two nearly 500 m deep boreholes cutting pleistocene fluvial sequences in the eastern part of the pannonian basin hungary have been studied magnetostratigraphy indicates that the sediment sequences span the last 2 6 ma without significant hiatuses comparison of systematic variations in magnetic susceptibility with changes in grain size distribution revealed milankovitch scale cyclicity in sedimentation of 40 and 100 kyr cyclicity which also shows a correspondence with the 18o variations of the odp 677 site comparison of pollen and gastropod paleoecological data with the magnetic susceptibility proxies suggests that climate was a major allogenic control on the cyclicity zones of high magnetic susceptibility correspond closely with sandy and coarse silty parts of the borehole successions which largely coincide with pollen and molluscs indicating a warm climate on the contrary zones with low magnetic susceptibility correspond to silts and clays and in most cases coincide with cold climate indicator pollen and molluscs heavy mineral and xrd studies revealed that magnetic susceptibility reflects the absolute amount of detrital fe oxide minerals mostly magnetite variations in the amount of magnetite are related to changes in heavy mineral content which arise primarily from changes in sediment supply to the basin by rivers the depositional model for the pleistocene fluvial sediments suggests that sediment flux increased towards the basin during interglacials due to the increased discharge and transport capacity of the rivers while a decreased sediment supply to the distal parts of the basin occurred during glacials a delay between erosion and storage of weathering products near to the source area and subsequent transportation and re deposition of sediments in the basin interior setting is an important cause of this effect

electromagnetic studies in the fennoscandian shield electrical conductivity of precambrian crust abstract electromagnetic em investigations of the 1980s in the fennoscandian baltic shield produced an unique and unified em data set studies include regional investigations by the magnetovariational mv method with large lateral sampling distance investigations of anomalous conductivity structures by magnetotelluric mt soundings and other em and electrical methods audio mt soundings d c dipole dipole and vlf resistivity profilings with shorter sampling distance and studies of the near surface conductivity by airborne em surveys the variety of methods provide an ability to map efficiently crustal conductivity structures from a regional scale of hundreds of kilometres down to local details of some metres in the anomalous structures the precambrian of the fennoscandian shield is characterized by roughly nw se directed elongated belts of conductors which separate more resistive crustal blocks the latter serve as transparent windows through which to probe deep electrical structure and belts of conductors as tectonic markers of ancient orogenic zones including 1 the kittila vetrenny poyas conductor 2 the lapland granulite belt and inari pechenga imandra varzuga conductors 3 the archaean proterozoic boundary conductor and 4 the southern finland conductor the conductive belts orogenic conductors indicate places where crustal masses collided and were finally sealed together enhanced conductivity in the orogenic conductors is caused primarily by an electronic conducting mechanism in graphite and sulphide bearing metasedimentary rocks estimations of the lower crustal conductivity indicate a laterally heterogeneous lower crust in the fennoscandian shield archaean lower crust seems to be in general more resistive than the early proterozoic lower crust of the karelian and svecofennian domains the lower crust in the southwestern part of the svecofennian domain and in the sveconorwegian domain seems to be more resistive than in the central part of the svecofennian domain