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

is the proterozoic ladoga rift se baltic shield a rift the southern part of the baltic shield hosts a series of mafic dykes and sills of mesoproterozoic ages including a ca 1 53 1 46 ga sheet like gabbro dolerite sills and the salmi plateau basalts from the lake ladoga region based on chiefly geochemical data the region is conventionally interpreted as an intracratonic ladoga rift graben we question the validity of this geodynamic interpretation by analyzing regional geophysical data crustal structure heat flow bouguer gravity anomalies magnetic anomalies and mantle vs velocities we provide a complete list of tectonic magmatic and geophysical characteristics typical of continental rifts in general and demonstrate that except for magmatic and perhaps some gravity signature the lake ladoga region lacks any other rift features we also compare the geophysical data from the lake ladoga region with similar in age midcontinent and valday rifts and provide alternative explanations for mesoproterozoic geodynamic evolution of the southern baltic shield we propose that mesoproterozoic mafic intrusions in southern fennoscandia may be associated with a complex deformation pattern during reconfiguration of a part of nuna columbia supercontinent which led to magma intrusions as a series of mafic dykes along lithosphere weakness zones and ponding of small magma pockets within the cratonic lithosphere consequent magma cooling and its partial transition to eclogite facies could have led to the formation of a series of basement depressions similar to intracratonic basins of north america while spatially heterogeneous thermo chemical subsidence with phase transitions locally speeded by the presence of subduction related fluids could have produced a series of faults bounding graben like structures

re os and 40ar 39ar isotope measurements of inclusions in alluvial diamonds from the ural mountains constraints on diamond genesis and eruption ages the re os isotope data for 20 syngenetic sulphide inclusions recovered from 15 diamonds and the 40ar 39ar laser probe eruption ages of 7 syngenetic clinopyroxenes recovered from 5 diamonds all from alluvial placer deposits in the ural mountains have been determined six eclogitic sulphide inclusions two of which coexist in the same diamond yield an isochron age of 1280 310 ma 2 with an unusually high initial 187os 188os ratio of 2 10 0 58 2 the age is interpreted to date remobilisation of carbon and sulphur and homogenisation of os during rift related thermal magmatic events that affected the east european craton eec at ca 1 3 ga the high initial os ratio suggests re os evolution over a 100 to 500 ma period within previously metasomatised lithosphere most likely the eec five eclogitic clinopyroxenes recovered from four diamonds yielded similar 40ar 39ar ages averaging 472 28 ma which likely approximate the time of source kimberlite lamproite eruption this age suggests that the ural diamonds are not likely to have derived either from the well known diamond bearing kimberlites of the siberian craton nor from presently known russian and finnish kimberlite provinces on the eec the urals placer deposits are mainly confined to 397 407 ma sedimentary rocks along the western side of these mountains with sediment transportation at that time generally from the north west present evidence suggests the existence of an undiscovered kimberlite lamproite source probably on the volgo uralia crustal segment of the eec which gave rise to the urals diamond deposits

reappraisal of deep seismic reflection profile viii across the pripyat trough two deep 24 s multi channel seismic reflection profiles each 200 km in length were shot across the pripyat trough a late devonian rift basin in belarus in the 1980s by the polessian geophysical expedition of the former soviet union additional post stack processing and migration on one of these profile viii has been applied at uppsala university and allows a re evaluation of the crustal structure of the pripyat trough with particular focus on the deeper parts previous interpretations showed numerous steeply dipping faults extending from the surface to depths of 30 40 km and sometimes penetrating into the mantle which was defined to be located at the top of a reflective zone beginning at about 10 s twt this reflective zone recorded to depths of 16 s twt was interpreted as a mixture of crustal and mantle rocks based on the original stacked section and the further processing the reflective zone is reinterpreted to be reflective lower crust not upper mantle and the moho is identified to be at 45 50 km about 15 s twt the major basin bounding faults appear to be listric at depth and sole into the top of the reflective lower crust rather than be truncated by it the newly interpreted crustal structure is consistent with a model of continental rifting involving brittle failure in a thick upper crustal layer and ductile shear in the lowermost crust with some degree of detachment between crust and mantle within the lower crustal ductile layer

the deformation history of the eastern part of the southwest swedish gneiss belt abstract the deformation history of the eastern part of the sw swedish gneiss belt has been investigated regional events more than c 1700 ma are identified as d1 d2 more than c 1400 ma as d3 less than c 1130 ma as d4 and more than c 900 ma as d5 d7 thus the d4 d7 events are considered to be sveconorwegian a common deformation history is suggested for southwestern sweden gradational changes in lithology from west to east are apparently associated with different tectonic styles and intensity of strain the principle deformation of the protogine zone of central sweden is suggested to be d4 the mylonite zone of western sweden appears to have been initiated before d3 the smaland varmland granitoid belt c 1700 ma is geneally considered as early postorogenic in respect to the svecokarelian orogeny because it has been influenced by the d3 event it represents a link between svecokarelian rocks and the sw swedish gneisses

the sarmatian crustal segment precambrian correlation between the voronezh massif and the ukrainian shield across the dniepr donets aulacogen abstract sarmatia is the southernmost crustal segment of the east european precambrian craton the phanerozoic dniepr donets aulacogen dda subdivides it into unequal halves the ukrainian shield in the south and the voronezh massif in the north correlation between the ukrainian shield and the voronezh massif is reviewed the overall structure and age province subdivision of sarmatia are assessed and constraints are imposed on displacement along the dda it is shown that differences in the trends of the principal basement structures highlighted by magnetic anomalies outlining palaeoproterozoic banded iron formations are a consequence of the heterogeneous structure of the archaean palaeoproterozoic basement rather than the kinematics of the opening of the dda only minor right lateral displacement along the dda has occurred the main crustal units of the ukrainian shield i e the azov middle dniepr and ingul ingulets blocks correlate excellently with the oskol sumy and sevsk blocks of the voronezh massif which allows the definition of three coherent oskol azov sumy dniepr and sevsk ingulets crustal domains presently available radiometric datings show that these three domains all have archaean crustal ages of 3 65 3 0 3 2 3 1 and 3 1 2 8 ga respectively the inter domain boundaries appear largely to have existed already in the archaean but were subsequently masked by palaeoproterozoic deformation and overlain by proterozoic cover and nappe piles amongst the three crustal domains of central and eastern sarmatia the oskol azov and sevsk ingulets ones have been reworked substantially in the palaeoproterozoic while the sumy dniepr domain acted as a stable cratonic unit although apparently rotating anti clockwise between 2 3 and 2 1 ga ago the palaeoproterozoic banded iron belts of sarmatia can be subdivided into the cratonic margin krivoy rog type and the interior intra domain oskol type the former outline the cratonic sumy dniepr domain while the latter associate with nw se continental rifting that followed archaean zones of weakness inside the oskol azov domain the bryansk block in the northwestern part of the voronezh massif is a part of the large bryansk bragin granulite domain which was formed between ca 2 2 and 2 1 ga ago and extends into the northwestern ukrainian shield in the east the lipetsk losev volcanic belt and the east voronezh turbidite schist province follow the boundary between the sarmatian and volgo uralian crustal segments together these two crustal units appear to represent a palaeoproterozoic orogenic environment ca 2 1 2 0 ga in age

magnetic anomaly maps from 40 n to 83 n derived from magsat satellite data stringent selection criteria using kp indices local magnetic activity estimates and visual intercomparisons followed by a statistical screening procedure have enabled the production of preliminary scalar and vector component magnetic anomaly maps derived from magsat data at northern high latitudes in spite of frequent external field disturbances comparisons between the maps and with other magnetic maps and geological information confirm their general validity

a model of the distribution of crustal magnetization in central europe compatible with the field of magnetic anomalies deduced from magsat results summary magsat data have been used to design scalar magnetic anomaly maps in an area that covers about half of europe on both sides of the north south directed european geotraverse egt the two maps refer to different altitudes and are intended to place recent compilations of aeromagnetic surveys along the egt wonik et al 1992 into global perspective the presentation of the magnetic anomalies and their compatibility with the aeromagnetic data required i a suitable reference field for subtraction from the measured total field which was derived from the field model m102089 by cain and ii a proper function of time reduction to the epoch 1980 0 which has been obtained from the actual satellite data because of the considerable noise in the data only those anomalies have been chosen for mapping which show up in all magsat tracks independent of altitude and epoch of measurement the anomaly maps are based upon the less disturbed dawn track data but a few dusk tracks have also been used as tie lines the anomalies are interpreted in terms of a magnetization model which divides the earth s lithosphere in europe into blocks of extension 1 n s by 2 e w and attributes to each block the depth integrated magnetization of the magnetized layer i e the magnetization integrated over the vertical extension of the block each block is represented by a dipole parallel to the inducing core field and the calculated model field is adjusted to the anomaly maps by trial and error starting with an initial coarse distribution that seems plausible from the anomalies and from local geomagnetic surveys for a realistic model field in the egt area a rather large portion of the earth s magnetized lithosphere outside the area under investigation must be included in the calculation in order to check whether the features of the magnetization model are necessary for the interpretation of the field anomalies they are compared with the distribution of magnetization that is obtained by the application of linear inverse theory prominent features of the model of crustal magnetization are the contrast between high values over the east european platform and low values in the younger parts of europe maxima at the iron ore districts of kiruna and kursk a broad minimum centred at st petersburg and only small differences between the mediterranean sea and the adjacent continental areas there is a fairly good agreement of these anomaly maps with the corresponding maps presented by cain et al 1989a

riphean rifting and major palaeoproterozoic crustal boundaries in the basement of the east european craton geology and geophysics abstract the east european craton which occupies the northeastern half of the european continent is characterized by the presence of giant even transcratonic systems of riphean meso to neoproterozoic rifts aulacogens the largest of these riphean rift systems tend to follow the course of palaeoproterozoic sutures and junction zones that subdivide the craton into three crustal segments with autonomous development histories these segments named fennoscandia volgo uralia and sarmatia are marked by distinct t magsat anomalies genetically the intersegment junction zones represent different types of collisional and accretional interaction between the component crustal segments of the craton in accordance with this the riphean rift systems vary in width the presence or absence of moho uplifts and other properties like for instance heat flow values however all of them follow belts of weakly magnetized crust studded with local magnetic maxima due to the presence of mafic rocks associated with the rifting the rift system intersegment junction zone relationships in consequence similarly represent a range of variation and cannot be referred to one single model

seismic velocity structure across the fennoscandia sarmatia suture of the east european craton beneath the eurobridge profile through lithuania and belarus

palaeoproterozoic upb zircon ages from belorussia new geodynamic implications for the east european craton abstract the crust in the northwest of the east european craton has traditionally been regarded as part of an archaean nucleus in eastern europe recent correlation with the baltic shield in scandinavia however suggests the need of reappraisal to study this problem zircons from three boreholes in the basement of the russian platform in belorussia belarus have been dated by the u pb and pb pb methods the ages indicated by two u pb discordias are 1982 26 ma for a metadacite from the central belorussian belt of volcanic arc igneous rocks and 1800 7 ma for mafic granulite from the baltic belorussian granulite belt largely built up of mafic igneous rocks for another central belorussian metadacite a pb pb age of 1975 10 ma was obtained in previous stratigraphies of belorussia all three dated rocks have been considered to be important components of an archaean even early archaean crust analyses of abraded crystal cores of complex zircons in one sample and separated core and rim zircons in another do not suggest substantially older ages of the core materials together with previous ages of granitoids and charnockites which had however been explained as a result of late magmatism and metamorphism and therefore not characterizing the age of the crust our data indicate the dominance of palaeoproterozoic rocks in the basement of the northwestern part of the east european craton that adjoins the tornquist line boundary between central and northeastern europe in conjunction with the results of a finnish estonian study of similarly allegedly archaean but in actual fact proterozoic rocks in southern estonia our results suggest the existence of large palaeoproterozoic crustal belts extending all the way from the finnish gulf to eastern poland this implies a substantial revision of established concepts on the precambrian geology in the northeastern half of the european continent

the east carpathian crystalline mesozoic zone romania paleozoic amalgamation of gondwana and east european craton derived terranes abstract the alpine orogen of the eastern carpathians contains pre alpine terranes framed by younger rocks the geological data on these terranes are reviewed and interpreted in order to elucidate their geodynamic evolution during the paleozoic the rebra tulghes terrane shows a stratigraphic sequence similar to avalonia neoproterozoic platformal metasediments rebra group overlain by late neoproterozoic volcano sedimentary rocks pietrosu bistritei porphyroids and lower paleozoic cover tulghes group the bretila terrane made up of orthogneisses pelitic gneisses granites amphibolites and micaschists shows affinities with the basement of the east european craton the modern juxtaposition of the bretila terrane and the rebra tulghes terrane may be explained in terms of collision between the latter and the east european craton during the ordovician times as an effect of collision the bretila terrane was thrust over the rebra tulghes terrane and induced the occurrence of thrust faults within it a lower paleozoic emplacement of the east carpathian pre alpine thrust nappes is thus inferred

magnetic anomaly map for northern western and eastern europe all of the available data describing the earth s magnetic field in northern western and eastern europe c 3 106 data points are compiled and presented at a scale of 1 20 400 000 the compilation meets two requirements i that the total field intensity anomalies reflect a survey acquired at an altitude of 3000 m above m s l and during the same epoch 1980 0 and ii that the anomalies are residuals after subtracting the common reference field dgrf1980 as the data span many epochs geomagnetic data recorded at observatories and repeat stations were used to determine the best model for estimating the secular variation in all data up to 1980 this yielded magnetic anomalies with wavelengths up to the order of 2000 km the precision of the resultant anomalies amplitudes about 1700 nt to 8000 nt differ between 10 nt in central europe and 20 nt in the outer parts of the compilation the resultant anomaly map contains a wealth of information to geoscientists studying global tectonic processes and the deep crustal composition of major continental crusts it clearly shows the different anomaly patterns between the palaeozoic and precambrian crusts of central and eastern europe these anomalies are interpreted as lateral changes of magnetization in a horizontal plate a comparison between the low level compilation and maps derived from the magsat satellite at about 360 km altitude allows the description of the two different crust types in europe

pomeranian caledonides nw poland a collisional suture or thin skinned fold and thrust belt abstract a main goal of this study was to understand the character of the caledonian deformation front in pomerania nw poland and its relationship to the adjoining teisseyre tornquist zone ttz in the context of the avalonia baltica early palaeozoic collision since the pomeranian caledonides are concealed beneath 1 4 km thick platform cover of upper palaeozoic and mesozoic sediments we used a combination of potential field seismic and well data to investigate the basement architecture and structure of the lower palaeozoic rocks starting from a qualitative review of gravity and magnetic data we built a 2d gravity and magnetic model upon the polandspan pl1 5600 seismic reflection line and applied 3d gravity inversion for a depth to basement study using well tops a top lower palaeozoic horizon and lower palaeozoic isopach map were created we found out that the pomeranian caledonides represent a thin skinned fold and thrust belt involving ordovician and silurian sediments of the caledonian foreland basin the deformation front was developed due to the buttressing effect of a basement ramp occurring directly above the ttz the latter corresponds to a precambrian suture zone overprinted by successive extensional tectonics and buried beneath the foreland basin in northern poland the suture is defined by a remnant crustal keel still preserved underneath the ttz probably resulting from precambrian collision during formation of the rodinia paleocontinent we propose that the extension of the thor suture separating baltica and avalonia must exist west from the ttz probably in the rugen area in our interpretation the ttz represents an intra cratonic crustal discontinuity comparable to its nw prolongation the sorgenfrei tornquist zone that runs across the baltic sea sweden and denmark consequently the basement underlying the palaeozoic platform in nw poland may be considered to have been part of baltica in early palaeozoic times

evolution of the archaean aulian gneiss complex middle dnieper gneiss greenstone terrain ukrainian shield shrimp upb zircon evidence abstract the aulian gneiss complex agc forms the main part of the middle dnieper gneiss greenstone terrain ukrainian shield as domal blocks between greenstone belts the complex consists of three main units the oldest consists of interlayered tonalitic gneisses amphibolites and ultramafic schists and are grouped into the supracrustal aulian series the second unit comprises coarse grained tonalitic and trondhjemitic gneisses which formed as small intrusive bodies within aulian series rocks the third unit contains diverse migmatites and gneissic granitoids collectively named the dniepropetrovsk granitoids and forming the main component of the agc shrimp u pb geochronology has been undertaken on these units of the agc weighted mean 207 pb 206 pb 2 ages for different zircon types in each sample are as follows 1 aulian series tonalitic gneisses 3196 13 and 3079 9 ma coarse grained amphibolite metagabbro of tholeiitic composition 3181 5 ma fine grained melanoamphibolite metavolcanic of basalt komatiitic composition 3078 17 ma 2 small intrusions into the aulian series trondhjemitic gneiss 3122 2 and 3081 5 ma 3 dniepropetrovsk granitoids tonalite 3074 7 ma trondhjemite 3070 ma these results allow the following synthesis for the agc 3200 3180 ma ago supracrustal dacite basalt komatiite sequence was formed and then 3120 ma ago it was intruded by trondhjemites and tonalites at 3080 ma metamorphism of amphibolite facies and perhaps local in situ partial melting caused growth of new zircon in the older lithologies and was accompanied by intrusion of younger of tonalites and trondhjemites these u pb zircon results have not substantiated a previous interpretation of sm nd whole rock isotopic data that very early archaean 3900 ma mafic components are present in the aulian series of agc

rapakivi granite anorthosite magmatism a way of thinning and stabilisation of the svecofennian crust baltic sea basin in the palaeoproterozoic a 55 80 km thick layer of crust was formed in the baltic sea region during the svecofennian orogeny at 1 9 1 8 ga today the remaining crustal thickness is 50 65 km in the marginal parts of the 1 000 000 km2 svecofennian domain the moho depth reached 40 75 km which exceeds the 40 km crustal thickness of the southwestern edge of the karelian archaean domain to which svecofennia was accreted the 1 65 1 50 ga rapakivi magmatism of the fennoscandian province was limited to the svecofennian domain the rapakivi igneous structures are confined to areally isolated subprovinces which each have distinct 20 60 ma long age spans of formation the petrologic sequences of the subprovinces are alike although similar petrological events occur at different times in the various subprovinces the internal structure of a subprovince generally consists of a main igneous polyphase unit in a central position with smaller felsic intrusions as well as mafic dike swarms spread over the peripheries of the subprovince the rapakivi magmatism started in juvenile crust which was in a late stage of erosional thinning 150 300 ma after the period of maximal thickening the most extensive rapakivi igneous activities were associated with crustal thinning down to 45 50 km as a result the thinner marginal parts of svecofennia and the large interior rapakivi subprovinces were of similar thicknesses as the crust the primary thickness of the original crust was maintained only in areas void of rapakivi magmatism no major events destroyed the svecofennian and rapakivi related crustal structures subsequent to emplacement thus it can be concluded that the extensive rapakivi igneous activity substantially thinned and stabilised the overthickened portions of the svecofennian crust

spatial variability at shallow snow layer depths in central dronning maud land east antarctica spatial variability at swallow snow layer depths in central dronning maud land east antarctic

seismic and aseismic slip evolution and deformation associated with the 2009 2010 northern malawi earthquake swarm east african rift summary interferometric synthetic aperture radar insar measurements field observations and elastic modelling of the 2009 2010 karonga northern malawi earthquake swarm reveal widespread coseismic and localized post seismic deformation in a period of about 1 5 months starting on november 5 29 m 4 earthquakes struck the region culminating in an mw 6 peak event on december 19 the next few months were characterized by significant localized deformation with a very low seismic moment release we find a very good agreement between insar and field observations of surface ruptures our best fitted coseismic models indicate dip slip displacements on a fault dipping 40 to the southwest with maximum slip of about 120 cm at 3 5 km depth fault activity continued until 2010 august as shallow aseismic afterslip mostly above the maximum coseismic slip patches although the swarm occurred within a coastal plain covered by porous quaternary sediments and a high water table the effect of poroelastic relaxation on the post seismic deformation was found to be negligibly small in contrast with other recent earthquake swarms along the east african rift the karonga swarm shows no evidence for dike intrusion

ion microprobe u th pb isotopic studies of zircons from three early precambrian areas in the u s s r abstract ion microprobe u th pb dating of zircons from three polymetamorphic archaean terranes in the soviet union has provided some precise estimates of protolith and metamorphic ages which confirm that the ages inferred previously from conventional zircon u pb analysis are broadly correct the gneisses of the novopavlovsk complex intersected by drilling in the eastern ukrainian shield contain pyroxenite enclaves zircons in one pyroxenite give 3450 8 ma and in a nearby enderbite give 3441 10 ma two pyroxenites from another drill hole contain zircons as old as 3633 16 ma and 3640 11 ma respectively in addition to clearly metamorphic zircons interpreted to be 2901 34 ma and 2664 68 ma old in one and 2595 92 ma old in the other the tonalitic orthogneiss host to these two enclaves is 3609 5 ma old and was metamorphosed at 2826 16 ma the zircons in the pyroxenites are probably not primary but grew when the pyroxenites were incorporated into the magmatic precursors of the orthogneisses zircons in a leucocratic orthogneiss from the aulandjha block in the omolon massif soviet far east are interpreted to be mostly 3481 18 ma old but a minor metamorphic population gives 2667 29 ma a recently reported age of 3900 ma from the daldin series in the anabar shield in central siberia was not confirmed the zircon age of an enderbite from the daldin series is interpreted to be 3164 32 ma