3 d density model of the crust of southern and central finland obtained from joint interpretation of the svekalapko crustal p wave velocity models and gravity data 3 d density model of the crust of southern and central finland obtained from joint interpretation of the svekalapko crustal p wave velocity models and gravity data

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

crust mantle boundary in the central fennoscandian shield constraints from wide angle p and s wave velocity models and new results of reflection profiling in finland 1 our paper presents analysis of the crust mantle boundary in the central fennoscandian shield based on new p and s wave two dimensional velocity models of the sveka 81 sveka 91 and fennia wide angle reflection and refraction profiles and on results of a new seismic reflection experiment in finland finnish reflection experiment fire in this area the crust is extremely thick 50 60 km and the moho boundary is difficult to detect using methods based on interpretation of p waves only near vertical profiling and wide angle experiments however the s wave reflections from the moho boundary sms are frequently more pronounced in wide angle data than the p wave reflections pmp in order to infer the crust mantle transition we developed new p and s wave velocity models using reprocessing of the old data and compared them to record sections of collocated reflection profiles and to published values of vp and vp vs for the main types of lower crustal and upper mantle rocks on the basis of the lateral variations of vp vp vs and reflectivity in the lower crust and upper mantle three main types of the crust mantle boundary were distinguished the first type corresponds to eclogitized lower crust that overlies peridotitic upper mantle in this case the moho coincides with the lithological crust mantle boundary the second type corresponds to lower crust composed of mafic garnet granulites overlying the peridotitic upper mantle for which the moho and the lithologic crust mantle boundary coincide as well the third type corresponds to the mafic garnet granulites underlain by a layer of eclogitic upper mantle in the latter case the lithological crust mantle boundary is deeper than the moho

palaeoproterozoic accretionary processes in fennoscandia abstract accretionary processes contributed to major continental growth in fennoscandia during the palaeoproterozoic mainly from 2 1 to 1 8 ga the composite svecofennian orogen covers c 1 10 6 km 2 and comprises the lapland savo fennia svecobaltic and nordic orogens it is a collage of 2 1 2 0 ga microcontinents and 2 02 1 82 ga island arcs attached to the archaean karelian craton between 1 92 and 1 79 ga andean type vertical magmatic additions especially at c 1 89 and c 1 8 ga were also important in the continental growth the palaeoproterozoic crust is the end product of accretionary growth continental collision and orogenic collapse preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening the pirkanmaa belt represents a composite accretionary prism and other preserved palaeosubduction zones are identified in the gulf of bothnia and the baltic sea areas in the southern segment of the lapland savo orogen collision between the archaean continent lower plate and the palaeoproterozoic arc microcontinent assembly upper plate produced a special type of lateral crustal growth the archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision

three dimensional tomographic inversion of combined reflection and refraction seismic traveltime data summary a tomographic inversion method is presented for the determination of 3 d velocity and interface structure from a wide range of body wave seismic traveltime data types it is applicable to refraction wide angle reflection normal incidence and multichannel seismic data and is best suited to a combination of these that provides good independent constraints on seismic velocities and interface depths the inversion process seeks a layer interface minimum structure model that is able to explain the given data satisfactorily by inverting to minimize data misfit and model roughness norms simultaneously this regularized inversion and the use of smooth functions to describe velocities and depths allows the highly non linear tomographic problem to be approximated as a series of linear steps the inversion process begins by optimizing the fit to the data of a highly smoothed initial model in each subsequent step structure is allowed to develop in the model with successively greater detail evolving until a satisfactory fit to the data is obtained parameter uncertainties for the final model are then estimated using an a posteriori covariance matrix analysis smooth layer interface models are parametrized using regular grids of velocity and depth nodes from which spline interpolated interface surfaces and velocity fields are defined forward modelling is achieved using ray perturbation theory and a two point ray tracing method that is optimized for a large number of closely spaced shot or receiver points the method may be used to generate 1 and 2 d models from for example vertical seismic profile data or 2 d surveys in which the 3 d geometry of a survey is correctly accounted for the ability of the method to resolve typical target structures is tested in a synthetic salt dome inversion from a set of noisy traveltime data the model converges quickly to a well resolved final model from different starting models the application of this method to real data is demonstrated with a combined 3 d inversion of refraction and reflection data which provide p wave velocity constraints on the methane hydrate stability zone in the cascadia margin offshore vancouver island

full scale crustal interpretation of kokkola kymi kokky seismic profile fennoscandian shield the kokkola kymi deep seismic sounding profile crosses the fennoscandian shield in northwest southeast nw se direction from bothnian belt to wiborg rapakivi batholith through central finland granitoid complex cfgc the 490 km refraction seismic line is perpendicular to the orogenic strike in central finland and entirely based on data from quarry blasts and road construction sites in years 2012 and 2013 the campaign resulted in 63 usable seismic record sections the average perpendicular distance between these and the profile was 14 km tomographic velocity models were computed with jive3d program the velocity fields of the tomographic models were used as starting points in the ray tracing modelling based on collected seismic sections a layer cake model was prepared with the ray tracing package seis83 along the profile upper crust has an average thickness of 22 km average and p wave velocities vp of 5 9 6 2 km s near the surface increasing downward to 6 25 6 40 km s the thickness of middle crust is 14 km below cfgc 20 km in se and 25 km in nw but vp ranges from 6 6 to 6 9 km s in all parts lower crust has vp values of 7 35 7 4 km s and lithospheric mantle 8 2 8 25 km s moho depth is 54 km in nw part 63 km in the middle and 43 km in sw yet a 55 km long section in the middle does not reveal an obvious moho reflection s wave velocities vary from 3 4 km s near the surface to 4 85 km s in upper mantle consistently with p wave velocity variations results confirm the previously assumed high velocity lower crust and depression of moho in central finland

atmospheric warming threatens the untapped glacial archive of ortles mountain south tyrol ortles mountain 3905 m a s l south tyrol italy is the highest mountain of the eastern european alps and its upper glacier alto dell ortles presents a unique opportunity to obtain the first paleoenvironmental record from an ice core in this area to study the suitability of this glacier as a drilling site in 2009 we performed the first preliminary study of its glaciological characteristics at 3830 m a s l the maximum thickness is 75 m and lamination of the exposed ice layers is excellent down to bedrock firn and ice lenses were observed in a 10 m shallow core and the firn ice transition was below 24 m the seasonal chemical signal is clearly preserved only within the uppermost 2008 and 2009 snow firn a simple mass balance model the incipient smoothing of the chemical record and the observed ice lenses provide evidence that melting infiltration and refreezing cycles occurred within the firn layers formed before 2008 nevertheless the mass balance of the upper part of alto dell ortles was positive 800 mm a 1 during the last few years we suggest that an environmental history is likely to be well preserved only within the ice layers formed before 1980 when summer air temperature was 28c colder than today in this area clearly the continued warming trend predicted to occur over the next few decades and the consequent increase in frequency and or intensity of infiltration processes will endanger the preservation of the glacial archive conserved in the deep ice layers of ortles mountain

structure and composition of the crust and upper mantle of the archean proterozoic boundary in the fennoscandian shield obtained by joint inversion of receiver function and surface wave phase velocity of recording of the svekalapko array summary we present a 3 d model of absolute values of s wave velocities and vp vs ratios in the crust and upper mantle beneath the svekalapko temporary seismic array that covered the transition zone between archean and proterozoic domains in the precambrian fennoscandian shield the model was obtained using joint inversion of p wave receiver functions rayleigh phase velocities and traveltimes of waves converted from the 410 km discontinuity p wave receiver functions and traveltimes of ps waves converted from the 410 km boundary were estimated for 30 broad band stations of the svekalapko array and short period smallaperture ruksa array in russian karelia the phase velocities of rayleigh waves were taken from previous surface wave studies bruneton et al 2004a b for each station the different data sets were merged and inverted by simulated annealing method after that a 3 d s wave velocity model and distribution of vp vs ratio was obtained from 1 d velocity models using special interpolation technique the new 3 d seismic model demonstrates pronounced lateral variations of values of vs and vp vs ratio in the crust and uppermost mantle the depth to the moho boundary varies from 51 to 63 km in our model which agrees with the results of previous controlled source seismic studies in the region the moho boundary is overlain by a high velocity lower crust hvlc with high vs which is non uniform in composition and origin our study showed no systematic correlation between the lithosphere structure and tectonothermal age of archean and proterozoic crustal terrains in the study area the exposed archean proterozoic suture so called ladoga bothnian bay zone is not observed as a mega scale structure in the crust and upper mantle generally the archean proterozoic transition occupies a larger area in the lithosphere than it was thought earlier it is marked by a moho depression stretching to the north and by a zone of high vp and high vp vs in the mantle our results supports the theory that the fennoscandian shield was assembled as a result of extensive collisional accretion of island arcs and microcontinental blocks and shows that these processes were working both in archean and proterozoic

simultaneous velocity and interface tomography of normal incidence and wide aperture seismic traveltime data summary a tomographic inversion technique that inverts traveltimes to obtain a model of the subsurface in terms of velocities and interfaces is presented it uses a combination of refraction wide angle reflection and normal incidence data it simultaneously inverts for velocities and interface depths and it is able to quantify the errors and trade offs in the final model the technique uses an iterative linearized approach to the non linear traveltime inversion problem the subsurface is represented as a set of layers separated by interfaces across which the velocity may be discontinuous within each layer the velocity varies in two dimensions and has a continuous first derivative rays are traced in this medium using a technique based on ray perturbation theory and two point ray tracing is avoided by interpolating the traveltimes to the receivers from a roughly equidistant fan of rays the calculated traveltimes are inverted by simultaneously minimizing the misfit between the data and calculated traveltimes and the roughness of the model this smoothing regularization stabilizes the solution of the inverse problem in practice the first iterations are performed with a high level of smoothing as the inversion proceeds the level of smoothing is gradually reduced until the traveltime residual is at the estimated level of noise in the data at this point a minimum feature solution is obtained which should contain only those features discernible over the noise the technique is tested on a synthetic data set demonstrating its accuracy and stability and also illustrating the desirability of including a large number of different ray types in an inversion

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

lithological interpretation of crustal composition in the fennoscandian shield with seismic velocity data abstract in this study we report the results of an investigation of lithological interpretation of the crust in the central fennoscandian shield in finland using seismic wide angle velocity models and laboratory measurements on p and s wave velocities of different rock types the velocities adopted from wide angle velocity models were compared with laboratory velocities of different rock types corrected for the crustal pt conditions in the study area the wide angle velocity models indicate that the p wave velocity does not only increase step wise at boundaries of major crustal layers but there is also gradual increase of velocity within the layers on the other hand the laboratory measurements of velocities indicate that no single rock type is able to provide the gradual downward increasing trends thus there must be gradual vertical changes in rock composition the downward increase of velocities indicates that the composition of the crust becomes gradually more mafic with increasing depth we have calculated vertical velocity profiles for a range of possible crustal lithological compositions the finnish crustal velocity profiles require a more mafic composition than an average global continental model would suggest for instance on the sveka 81 transect the calculated models suggest that the crustal velocity profiles can be simulated with rock type mixtures where the upper crust consists of felsic gneisses and granitic granodioritic rocks with a minor contribution of amphibolite and diabase in the middle crust the amphibolite proportion increases the lower crust consists of tonalitic gneiss mafic garnet granulite hornblendite pyroxenite and minor mafic eclogite assuming that these rock types are present in sufficiently extensive and thick layers they would also have sufficiently high acoustic reflection coefficients for generating the generally well developed reflectivity in the crust in the central part of the shield density profiles calculated from the lithological models suggest that there is practically no density contrast at moho in areas of the high velocity lower crust comparison of reflectors from fire 1 and fire 3 transects and the velocity model from sveka 81 wide angle transect indicated that the reflectors correlate with velocity layering but the three dimensional structures of the crust complicate such comparisons

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

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

a priori evidence for zircon antecryst entrainment in megacrystic proterozoic granites entrainment of antecrystic zircon in early saturated main silicate phases has been recognized as a complicating factor in high precision u pb geochronology of phanerozoic granitic systems but has not been demonstrated for precambrian rocks we report u pb ages secondary ion mass spectrometry and trace element laser ablation inductively coupled plasma mass spectrometry compositions of zircon from three main ferroan granite types of the late paleoproterozoic wiborg rapakivi granite batholith of southeastern finland and vicinity zircon was extracted from the interior parts of megacrystic diameters as large as 15 cm rapakivi textured alkali feldspar ovoids using a microdrilling technique compared to zircon in ovoid groundmass zircon in the ovoids has higher incompatible element values e g uranium and rare earth elements and for two of the three samples examined slightly older by 1 m y average u pb ages these observations suggest that the ovoid material of the rapakivi granites entrained zircon from earlier magmas that were compositionally different than the final magmatic host of the ovoids groundmass zircon implies similar average u pb ages for all the studied samples ca 1628 ma this is regarded as the final crystallization age of the granites these observations show that it is viable to measure statistically valid age differences between antecrysts and groundmass of proterozoic granites overall our sampling technique provides enhanced textural control of antecrystic zircon in a wide range of sample materials

the moho depth map of the european plate summary the european plate has a 4 5 gy long and complex tectonic history this is reflected in the present day large scale crustal structures a new digital moho depth map is compiled from more than 250 data sets of individual seismic profiles 3 d models obtained by body and surface waves receiver function results and maps of seismic and or gravity data compilations we have compiled the first digital high resolution map of the moho depth for the whole european plate extending from the mid atlantic ridge in the west to the ural mountains in the east and from the mediterranean sea in the south to the barents sea and spitsbergen in the arctic in the north in general three large domains within the european plate crust are visible the oldest archean and proterozoic crust has a thickness of 40 60 km the continental variscan and alpine crust has a thickness of 20 40 km and the youngest oceanic atlantic crust has a thickness of 10 20 km

analog modeling of one way gravitational spreading of hot orogens a case study from the svecofennian orogen fennoscandian shield abstract geodynamic models offer insight into deformation styles of syn and post collisional extensional processes such models often ignore the effect of lateral viscosity contrast between accreted terranes and inherited tectonic boundaries this paper presents a set of crustal scale analog experiments of one way post collisional gravitational spreading testing the effect of crustal heterogeneity by using two mechanically contrasting blocks pre existing cuts with varying dips and three layers with contrasting rheologies two blocks represent strong mature and weak juvenile crustal parts both blocks have three horizontal crustal layers brittle upper weak partially molten middle and dry strong lower layer cuts represent pre existing crustal scale shear zones and tectonic boundaries the experiments indicated that tectonic boundaries may facilitate exhumation and increase extensional strain when the boundaries dip opposite to the flow direction the tectonic boundaries may also split and divide the crust into separate blocks with independent deformation signatures shortening elongation or rotation the tectonic boundaries rotated along vertical axes toward the flow direction the modeling results suggest that in areas with inherited tectonic boundaries and compositional differences the amount of extension is unevenly distributed between the different parts during the widespread unilateral gravitational spreading and that the flow has different effects on the contrasting crustal units in both lateral and vertical directions to validate the modeling the results are compared to geophysical and geological data from the paleoproterozoic svecofennian orogen in the fennoscandian shield the comparison suggests that the orogen underwent 50 extension and was thinned by maximum of 20 via westward gravitational spreading this spreading rotated the crustal scale blocks shear zones and tectonic contacts it also brought exhumed high grade middle crustal blocks into contact with subsided low grade upper crustal blocks we suggest that results of the analog models can be used as proxies in both ancient and modern hot accreted orogens which have undergone post convergence continental mid crustal weakening and have comprised of juxtaposed terranes with varying rheological compositions

new constraints for the source characteristics deposition and age of the 2 1 1 9 ga metasedimentary cover at the western margin of the karelian province abstract the paleoproterozoic cover sequence at the 100 150 km wide western margin of the archean karelian province is dominated by deep water lower and upper kaleva metasediments we present here an interpretation of sm nd isotope and geochemical data on 36 samples tims multi grain u pb zircon analyses on nine samples and ca 100 sims analysis of detrital zircon grains from four upper kaleva and one lower kaleva samples the lower kaleva is characterized by autochthonous parautochthonous lithologically heterogeneous metaturbidites showing common enrichment in quartz all the analysed detrital zircons are of a local neoarchean source but tdm variation up to 2 4 ga combined with geochemical data indicate abundant mixing of paleoproterozoic mafic material presumably from 2 1 ga plateau lavas and dykes in most of the lower kaleva samples the upper kaleva is dominantly allochthonous with tectonically enclosed fragments of ophiolite bodies and it is characterized by lithological and geochemical isotopic homogeneity geochemical isotopic and detrital zircon data favour material derived from an orogenic domain comprising both archean and proterozoic units followed by effective mixing during the transport the archean zircon grains 25 are mostly neoarchean the paleoproterozoic grains lack zircons at 2 5 2 2 ga and plot dominantly 92 between 1 92 and 2 05 ga the indicated maximum deposition ages vary from 1 95 1 94 ga to 1 92 ga the main source area proposed is the himalaya type lapland kola orogen now in the northeast which experienced mountain building and erosion at 1 95 1 91 ga the western margin of the karelian province shows evidence of rifting and lithosphere thinning from 2 1 to 1 95 ga but it is still under debate whether the craton breakup occurred at 2 06 ga in a volcanic or later at 1 95 ga in a non volcanic margin setting one hypothesis is that the onset of collision in the northeast changed plate motion and lead to a new spreading within the pre existing passive margin at 1 97 1 95 ga thus both a volcanic margin at 2 06 ga and a non volcanic margin at ca 1 95 ga could have been operated at the western margin of the karelian province

crustal architecture of the inverted central lapland rift along the hukka 2007 profile we have studied the lateral velocity variations along a partly buried inverted paleo rift in central lapland northern europe with a 2d wide angle reflection and refraction experiment hukka 2007 the experiment was designed to use seven chemical explosions from commercial and military sites as sources of seismic energy the shots were recorded by 102 stations with an average spacing of 3 45 km two dimensional crustal models of variations in p wave velocity and vp vs ratio were calculated using the ray tracing forward modeling technique the hukka 2007 experiment comprises a 455 km long profile that runs nnw sse parallel to the kittila shear zone a major deformation zone hosting gold deposits in the area the profile crosses paleoproterozoic and reactivated archean terranes of central lapland the velocity model shows a significant difference in crustal velocity structure between the northern distances 0 120 km and southern parts of the profile the difference in p wave velocities and vp vs ratio can be followed through the whole crust down to the moho boundary indicating major tectonic boundaries upper crustal velocities seem to vary with the terranes compositional differences mapped at the surface the lower layer of the upper crust displays velocities of 6 0 6 1 km s both paleoproterozoic and archean terranes are associated with high velocity bodies 6 30 6 35 km s at 100 and 200 350 km distances the central lapland greenstone belt and central lapland granitoid complex are associated with a 4 km thick zone of unusually low velocities 6 0 km s at distances between 120 and 220 km we interpret the hukka 2007 profile to image an old partly buried inverted continental rift zone that has been closed and modified by younger tectonic events it has structural features typical of rifts inward dipping rift shoulders undulating thickness of the middle crust high velocity lower crust and a rather uniform crustal thickness of 48 km

three stages to form a large batholith after terrane accretion an example from the svecofennian orogen abstract the magmatic history of batholiths formed as the result of tectonic accretion mirrors the evolution of the underlying crust from island arcs to accretion and collapse chemical composition age and structure give information about the magma source and emplacement depths the target of this study is the central finland batholith cfgc that provides a magmatic record after terrane accretion during the svecofennian orogeny followed by orogenic collapse the crustal source of partial melting has changed during the crustal evolution and differentiation after the terrane accretion and when the stress regime has changed from contraction to extension here we present a three stage magmatic evolution model for accretional batholith in the svecofennian orogen in the first stage the lower crust of the accreted crust melts partially and produces calcic tonalitic melts that are emplaced above the upper middle crustal detachment zone group 1 1887 3 ma in the second stage the middle crust begins to melt producing calc alkalic granodioritic melts that are emplaced close to the surface group 2 1890 1886 ma and at both sides of the upper middle crustal detachment zone group 2 1884 1883 ma in the third stage the lower crust remelts and produces bimodal calc alkalic gabbroic and alkali calcic to alkalic granitic melts group 3 1881 1880 ma which are emplaced above the upper middle crustal detachment zone this kind of crustal differentiation produces a mafic lower an intermediate middle and a felsic upper crust the third stage represents the last magmatic event of the svecofennian accretional orogeny where the high velocity lowermost crust is formed during this stage crustal thickening and widespread mid crustal partial melting have inititated lateral flow after or concurrently with the emplacement of the group 2 granodiorite the lateral flow has reactivated large scale shear zones allowing the transportation of the group 3 rocks from the lower crust to the upper middle crustal detachment zone the change in the degree of deformation at the exposed parts from undeformed through local to pervasive is attributed to differences in the exhumation level

a tomographic crustal velocity model of the central fennoscandian shield summary in this paper local seismic tomographic method is used to find the terrane distribution within the central parts of the accretionary svecofennian orogen this study presents a crustal pwave velocity model and for the first time the s wave velocity and vp vs ratio model of svekalapko area that is 700 800 km 2 in southern and central finland the 3 d model is presented as p and s wave velocity as well as vp vs ratio depth slices and vertical sections the data set comprised of several subsets of crustal pg and sg wave traveltime data from local events recorded by the svekalapko seismic tomography array in 1998 1999 from older controlled source shots recorded at portable stations as well as at permanent stations and from non controlled chemical explosions recorded at permanent seismic stations from 300 local explosions a total of 10 404 pg wave and 9361 sg wave arrival times were inverted to create independent 3 d vp and vs tomographic models from which the vp vs ratio was calculated according to sensitivity tests the optimal horizontal resolution of recognizable velocity structures in the central study area is at least 60 km down to depths of 40 km in the final model the seismic velocities vary smoothly the lateral variations are larger for vp 5 9 6 6 km s 1 than for vs 3 5 3 8 km s 1 in the upper 20 km of the crust at depths of 20 40 km vp varies from 6 5 to 7 2 km s 1 and vs from 3 7 to 4 1 km s 1 the vp vs ratio varies spatially more distinct than p and s wave velocities usually from 1 70 to 1 74 in the upper crust and from 1 72 to 1 78 in the lower crust schist belts and their continuations at depth are associated with lower velocities and lower vp vs ratios vp 6 2 6 8 km s 1 vs 3 6 3 9 km s 1 vp vs 1 68 1 73 than the granitoid areas vp 6 3 7 4 km s 1 vs 3 6 4 2 km s 1 vp vs 1 72 1 78 the svecofennian orogen was accreted from crustal blocks ranging in size from 100 100 km 2 to 200 200 km 2 in cross sectional area the intervening sedimentary belts have ca 0 2 km s 1 lower p and s wave velocities and ca 0 04 lower vp vs ratios tomographic model supports the idea that the thick svecofennian crust was accreted from several crustal terranes some hidden and that the crust was later modified by intra and underplating

petrogenesis of the post kinematic magmatism of the central finland granitoid complex ii sources and magmatic evolution the 188 187ga post kinematic granitoids of the central finland granitoid complex cfgc provide a key geochemical link to understanding granite formation in paleoproterozoic orogenic and post orogenic terrains thickness of the crust and intra crustal differentiation processes played an important role in the formation of three granitoid types that shortly followed the peak of the svecofennian orogeny in the eastern cfgc pyroxene bearing plutons with c type geochemical affinities predominate these were formed from a mixture of low to moderate degree partial melts 30 of mafic mantlederived basaltic 49 sio2 source rocks and partial melts of pre existing mafic granulite lower crust at depth in the western cfgc high silica iron rich fluorite bearing plutons with a type granite characteristics predominate a higher thermal gradient thinner upper and lower crust and significantly more shallow moho depth resulted in higher proportions of crustal melts 03 0 4v s 0 1 02 in the eastern cfgc incorporated into the partial melts of a mafic mantle derived source a geochemical model focusing on the j and honkajoki plutons of the post kinematic suite is presented constraining the nature of the source rock s the degree of partial melting proportions of partial melts and fractionation processes involved

petrogenesis of early permian sanukitoids from west junggar northwest china implications for late paleozoic crustal growth in central asia abstract sanukitoids and their equivalents are rare subduction related rock types that have been found in modern arc settings and in late archean sequences the investigation of sanukitoids is of particular importance to a better understanding of crust forming processes and continental growth in this paper we report zircon u pb and ar ar ages and major element trace element and sr nd hf isotope data for the bieluagaxi dioritic pluton and dikes from the southern part of the west junggar nw china these rocks formed in the early permian and show a remarkable geochemical affinity with cenozoic sanukitoids of the setouchi volcanic belt of sw japan with high mg 48 73 values cr 54 539 ppm ni 21 197 ppm contents and th la 0 15 0 37 ratios and low sr y ratios 16 27 and sr 263 442 ppm contents they may be generated by the partial melting of subducting sediments and subsequent melt mantle interaction additionally the sanukitoids are also widespread in the karamay baogutu area of west junggar accompanied by high sr average 713 ppm contents and sr y 50 130 ratios and low y 6 9 12 6 ppm contents the difference in petrochemical characteristics between the baogutu karamay and bieluagaxi sanukitoids can be explained by the difference in depth of initial melting origin composition and fractional crystallization the baogutu karamay sanukitoids were probably formed under eclogitic conditions while the bieluagaxi sanukitoids were at a shallower depth moreover the compositional similarity between continental crust forming rocks and the bieluagaxi sanukitoids suggests that the sanukitoids genesis could be closely related to the process of continental crust formation the late carboniferous early permian sanukitoids in the west junggar may be an indicator of anomalous thermal activity ridge subduction may play a crucial role in the evolution and growth of the continental crust in central asia

evolution of a palaeoproterozoic giant magmatic dome in the finnish svecofennian new insights from u pb geochronology abstract the svecofennian domain comprises a c 800 km by 800 km area in finland and sweden mainly composed of palaeoproterozoic granitoids schists and gneisses in the central part of the domain in western finland the vaasa complex hosts a sequence of high grade metamorphic rocks and granites formed by crustal anatexis during the culmination of the 1 9 ga svecofennian orogeny the vaasa complex consists of a c 8000 km 2 granitoid batholith surrounded by diatexitic and metatexitic migmatites as well as metasedimentary rocks in order to enhance the perception of the crystallisation history provenance and maximum sedimentation age of the protoliths involved nine granitoid xenolith pairs and a palaeosome neosome pair were collected from the main body of the vaasa batholith and surrounding rocks and zircon from them was u pb dated using the la mc icpms method the u pb zircon crystallisation ages of the granitoids are 1 88 1 87 ga which is in accordance with the few previous results from the vaasa region and confirms a relatively swift crystallisation of the granite core throughout the finnish part of the complex the samples from the central part of the batholith have younger zircon ages than those from batholith margin the inherited zircon age distribution pattern of the granitoids resembles that of the metasedimentary xenoliths our new comprehensive data combined with whole rock geochemistry from previous studies render an in situ origin of the vaasa granite probable the maximum sedimentation age of the supracrustal precursors of the vaasa complex is 1 92 ga some of the metasedimentary xenoliths from the vaasa complex host zircon that is coeval with their host granitoids and register incipient melting based on these 1 88 1 86 ga zircon grains the maximum sedimentation age would be deemed too young therefore our data calls for caution when determining maximum sedimentation ages of high grade metasedimentary rocks