how does orogenic crust deform evidence of crustal scale competent behaviour within the partially molten middle crust during orogenic compression abstract granitic partial melts are generally thought to significantly weaken the orogenic crust and ultimately lead to the collapse of an orogen studies from different orogens have shown that the syn melt deformation behaviour at the orogenic scale is however more complex in addition once fully crystalline granitic material strengthens the crust linking the evolution from melt present to melt absent deformation at the scale of the orogen is a challenging but necessary task if one wishes to investigate the overall behaviour of the middle crust in this paper we make observations of orogen scale deformation developed over a period of 30 40 ma during crustal partial melting of the middle and lower crust of a palaeoproterozoic orogen the crust shows a globally common pattern where coeval partial melting compressional deformation and transtensional structures co exist we demonstrate this complex interaction through an integrated approach using multiple datasets the key observation is that of widespread regular orogen scale shortening by folding of sub horizontal anatectic granitic sheets and highly migmatized lithologies this shortening interacted with modest orogen parallel lateral stretch expressed as local extensional transtensional structures the pervasive and dominant deformation style is that of folding of the syn kinematic granite sheets and voluminous migmatites this demonstrates that the principal deformation style of deformation within the partially molten mid crust is competent rather than weak the observed evidence of weak behaviour such as strike slip or transtensional shear zones accommodating lateral escape are localized in comparison and not necessarily associated directly with the granite sheets or the highly migmatized volumes of the crust moreover the strain intensity fold wavelengths and amplitudes seems independent of the overall melt fraction the implications are that while widespread volumes of partial melt will weaken the crust overall compared to the brittle upper crust and while individual melt bodies can possibly persist for up to some millions of years in the middle crust i the relative strength between partially molten volumes is not primarily controlled by melt fraction and ii individual melt volumes may be too short lived at the time scale of orogenic deformation i e orogenic strain rates to significantly influence the overall deformation style

seismic imaging of subduction zone metamorphism combined analysis of high resolution seismic images of the alaska and cascadia subduction zones reveals where metamorphic fl uids are released both images show the subducted oceanic crust as a dipping low velocity layer with a clear termination depth however in alaska the crust is thicker 15 20 km compared to 8 km and terminates at greater depth 120 km com pared to 40 km than in cascadia based on metamorphic reaction estimates and geodynamic models we demonstrate that the termination depth corresponds to eclogitization of the crust triggered by dehydration of water bearing minerals and that the location of this reaction is dependent on the thermal structure of the subducted slab

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

imaging subduction from the trench to 300 km depth beneath the central north island new zealand with vp and vp vs summary recent dense deployments of portable digital seismographs have provided excellent control on earthquakes beneath the central north island of new zealand here we use a subset of the best recorded earthquakes in an inversion for the 3 d vp and vp vs structure the data set includes 39 123 p observations and 18 331 s observations from 1239 earthquakes and nine explosions the subducted plate is imaged as a high vp low vp vs feature vp within the mantle of the subducted slab is almost always 8 5 km s 1 which requires the ca 120 myr slab to be unusually cold the low vp vs within the subducted plate closely parallels the lower plane of the dipping seismic zone it most likely indicates fluid resulting from dehydration of serpentine in the slab mantle and the earthquakes themselves are likely to be promoted by dehydration embrittlement we identify a region with vp 8 0 km s 1 directly above the dipping seismic zone can be interpreted as sinking entrained with the motion of the subducted slab and forming a viscous blanket that insulates the slab from the high temperature mantle wedge material in the overlying low vp region can be interpreted as rising within a return flow within the wedge the volcanic front appears to be controlled by where this dipping low vp region meets the base of the crust the thickness of the backarc crust also shows significant variation along strike in the central taupo volcanic zone tvz the crust is ca 35 km thick while southwest of mt ruapehu the crust thickens by ca 10 km there is no significant low vp zone in the mantle wedge in this southwestern region suggesting that this thicker crust has choked off mantle return flow the seismic tomography results when combined with constraints on mantle flow from previous shear wave splitting results provide a plausible model for both the distribution of volcanism in the central north island and the exceptional magmatic productivity of the central tvz

thermal structure beneath tohoku northeast japan combined with geophysical observations and petrological considerations we have studied the thermal models of the tohoku northeast japan subduction zone the temperature of the deeper part of the mantle wedge is constrained by the petrological requirements that demand a high temperature region 1400 c beneath the volcanic zone to satisfy this condition namely to explain the arc volcanism in the tohoku subduction zone a high temperature upper mantle beneath the japan sea possibly 1400 c at the depth shallower than 100 km is required heat flow distribution which constrains the temperature structure at shallower depth shows that low heat flow values ranging from 20 to 50 mw m2 are distributed between the trench and aseismic front and consistently high heat flow values 80 120 mw m2 prevail in the area landward of the volcanic front including the japan sea recent heat flow measurements indicate that the transition from low to high heat flow occurs between the aseismic front and volcanic front this heat flow distribution restricts the form of the flow in the mantle wedge and shear stress acting at the plate boundary a rigid non moving triangular region bounded by the subducting slab and the vertical plane beneath the aseismic front is necessary to explain the low heat flow zone between the trench and aseismic front shear stress of about 500 to 1000 bar is supposed to act at the interface between the subducting slab and rigid triangular mantle we also suggest that to explain the peak of heat flow near the volcanic front 120 mw m2 magma migration rate of about 0 05 cm3 cm 2 year 1 is necessary a comparison of the thermal structure of the tohoku subduction zone with that of the central south america which was preliminary calculated by the similar scheme shows that the temperature in the mantle wedge of the central south america is probably lower than that of the tohoku subduction zone this difference may result in that of the nature of volcanic rocks

the early proterozoic orijarvi triangle southwest finland a key area on the tectonic evolution of the svecofennides abstract a consistent pattern of three successive deformation phases is present in the west uusimaa area in the south svecofennian schist belt of finland in which the orijarvi triangle bordered by shear zones forms a low strain area it is shown that the well preserved early folds and foliation in the orijarvi triangle correlate with the regionally oldest and completely transposed structures a model is given for the tectonic development of the west uusimaa area involving 1 an early phase of subhorizontal probably thrust related folding 2 a second phase of crustal shortening and 3 a late phase of folding and formation of vertical shear zones the first two phases took place during prograde metamorphism whereas the last one records retrogression shear zones were probably active during all phases subhorizontally thrusting during d 1 transversely wrenching during d 2 and vertically uplift during d 3 it is suggested that the d 1 d 2 sequence represents a progressive deformation in a plate collision system whereas d 3 is related to a 1533 23 ma old post svecofennian possibly gothian event a model is presented in which the development and preservation of the orijarvi triangle is explained by the local development of transversal shear zones along anatectic plutons during d 2 these shear zones were then reactivated during d 3 the model presented implies that the sigmoidal pattern present in southern finnish geology was caused during a late stage of the d 2 crustal shortening event along three broad parallel sinistral shear zones it is suggested that these shear zones represent deep seated parts of wrench faults in a complex plate tectonic system involving several island arcs and that they were caused by differential movement of the south svecofennian schist belt with respect to the tampere schist belt

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

unusual lithospheric structure beneath the hyderabad granitic region eastern dharwar craton south india abstract using multiparametric geological and geophysical data the evolutionary history of the lithosphere beneath the late archean early proterozoic hyderabad granitic region hgr of the eastern dharwar craton of south indian shield has been attempted our study reveals that the entire granitic region and the surroundings 200 km 200 km is being neotectonically uplifted possibly due to a major intrusive body situated at sub crustal depth leading to changes in river courses and also erosion of several kilometers 10 km from its upper crustal column beneath this region the moho is elevated 32 33 km the reduced heat flow is of the order of 28 mw m2 and the asthenosphere is located at shallow depths of about 124 km i e far less than 200 400 km depth found in precambrian shield areas of the globe the cratonic mantle lithosphere beneath hgr contains a highly conductive 15 m hydrous and seismically anisotropic metasomatic zone between depth of 90 and 100 km where the estimated temperatures could be in the range of 900 1000 c the surface granitic layer containing unusually high radioactivity 5 25 w m3 appears very thin probably a kilometer or so beneath which the entire crustal column seems to be differentiated and made up of low radioactive granulite facies rocks of acid to intermediate composition persistent episodic thermal reactivations during the last 2 6 gega years and continuous uplifting seems to have played a major role towards making its crustal column unusual

1 8 ga svecofennian post collisional shoshonitic magmatism in the fennoscandian shield abstract at least 14 small 1 11 km across 1 8 ga svecofennian post collisional bimodal intrusions occur in southern finland and russian karelia in a 600 km long belt from the aland islands to the nw lake ladoga region the rocks range from ultramafic calc alkaline apatite rich potassium lamprophyres to peraluminous hibasr granites and form a shoshonitic series with k 2 o na 2 o 5 k 2 o na 2 o 0 5 al 2 o 3 9 over a wide spectrum of sio 2 32 78 although strongly enriched in all rocks the lile ba and sr and the lree generally define a decreasing trend with increasing sio 2 depletion is noted for hfse ti nb and ta available isotopic data show overlapping values for lamprophyres and granites within separate intrusions and a cogenetic origin is thus not precluded initial magmas mg 65 in this shoshonitic association are considered to be generated in an enriched lithospheric mantle during post collisional uplift some 30 ma after the regional svecofennian metamorphic peak however prior to the melting episode the lithospheric mantle was affected by carbonatite metasomatism more extensively in the east than in the west the melts generated in the more carbonate rich mantle are extremely enriched in p 2 o 5 4 f 12 000 ppm lile ba 9000 ppm sr 7000 ppm lree la 600 ppm and ce 1000 ppm the parental magma underwent 55 60 fractionation of biotite clinopyroxene apatite magnetite sphene whereupon intermediate varieties were produced after further fractionation 60 80 of k feldspar amphibole plagioclase minor magnetite sphene and apatite leucosyenites and quartz monzonites were formed in the west where the source was less affected by carbonatite metasomatism calc alkaline lamprophyres vogesites minettes and spessartites and equivalent plutonic rocks monzonites were formed removal of about 50 of biotite amphibole plagioclase magnetite apatite and sphene produced peraluminous hibasr granites the impact of crustal assimilation is considered to be low at about 1 8 ga the post collisional shoshonitic magmatism brought juvenile material particularly enriched in alkalis lile lree and f into the crust although areally restricted the regional distribution of the post collisional intrusions may indicate that larger volumes of 1 8 ga juvenile material resides in unexposed parts of the crust

effect of hydrothermal circulation on slab dehydration for the subduction zone of costa rica and nicaragua abstract dehydration of subducting oceanic plates is associated with mantle wedge melting arc volcanism intraslab earthquakes through dehydration embrittlement and the flux of water into the mantle in this study we present two dimensional thermal models of the costa rica nicaragua subduction zone to investigate dehydration reactions within the subducting cocos plate seismic and geochemical observations indicate that the mantle wedge below nicaragua is more hydrated than that below costa rica these trends have been hypothesized to be due to a variation in either the thermal state or the hydration state of the subducting slab despite only small variations in plate age along strike heat flow measurements near the deformation front reveal significantly lower heat flow offshore nicaragua than offshore costa rica these measurements are interpreted to reflect an along strike change in the efficiency of hydrothermal circulation in the oceanic crust we parameterize thermal models in terms of efficient and inefficient hydrothermal circulation and explore their impact on slab temperature in the context of dehydration models relative to models without fluid flow efficient hydrothermal circulation reduces slab temperature by as much at 60 c to depths of 75 km and increases the predicted depth of eclogitization by 15 km inefficient hydrothermal circulation has a commensurately smaller influence on slab temperatures and the depth of eclogitization for both regions the change in eclogitization depth better fits the observed intraslab crustal seismicity but there is not a strong contrast in the slab thermal structure or location of the main dehydration reactions consistent with other studies these results suggest that observed along strike differences in mantle wedge hydration may be better explained by a northwestward increase in the hydration state of the cocos plate before it is subducted

petrological and geochronological constraints on lower crust exhumation during paleoproterozoic eburnean orogeny nw ghana west african craton we present new petrological and geochronological data on high grade ortho and paragneisses from northwestern ghana forming part of the paleoproterozoic 2 25 2 00 ga west african craton the study area is located in the interference zone between n s and ne sw trending craton scale shear zones formed during the eburnean orogeny 2 15 2 00 ga high grade metamorphic domains are separated from low grade greenstone belts by high strain zones including early thrusts extensional detachments and late stage strike slip shear zones paragneisses sporadically preserve high pressure low temperature hp lt relics formed at the transition between the blueschist facies and the epidote amphibolite sub facies 10 0 14 0 kbar 520 600 c and represent a low 15 c km 1 apparent geothermal gradient migmatites record metamorphic conditions at the amphibolite granulite facies transition they reveal a clockwise pressure temperature time p t t path characterised by melting at pressures over 10 0 kbar followed by decompression and heating to peak temperatures of 750 c at 5 0 8 0 kbar which fit a 30 c km 1 apparent geotherm a regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise p t t path characterised by peak metamorphic conditions of 7 0 10 0 kbar at 550 680 c which match a 20 25 c km 1 apparent geotherm these p t conditions were reached after prograde burial and heating for some rock units and after decompression and heating for others the

geochronology and structural relationships of mesothermal gold mineralization in the palaeoproterozoic jokisivu prospect southern finland the palaeoproterozoic svecofennian orogen in southern finland contains a number of orogenic gold occurrences the jokisivu gold deposit comprising auriferous quartz veins is hosted by syn tectonic quartz diorites to gabbros mineralization occurs in approximately wnw ese and wsw ene trending shear zones which probably branch from regional scale nw se trending shears ore zone fabrics post date regional scale folding and the metamorphic peak and can be correlated with late svecofennian regional shear tectonics d 6 1 83 1 78 ga indicating that mineralization formed during the late stages of orogenic evolution sims and tims u pb dating of three samples place tight constraints on the age of gold mineralization zircons from both unaltered and altered quartz diorites have ages of 1884 4 ma and 1881 3 ma respectively these are interpreted as the crystallization age of the rock and as providing the maximum age for mineralization zircon rims from an altered quartz diorite from the ore zone give ages of c 1802 15 ma which overlap with the 1801 18 ma titanite mean pb pb age from the ore zone the ages are similar to the age of the pegmatite dyke that cuts the ore zone and whose zircon age of 1807 3 ma is approximately the same as the 1791 2 ma monazite age tims giving the minimum age of the gold mineralization the mineralization and its structural framework can be correlated with coeval late svecofennian shear tectonics related to wnw ese oriented shortening in southern finland extensive c 1 8 ga granite magmatism shear zone development and associated gold mineralization are of regional importance also in the northern and western fennoscandian shield finnish lapand and sweden a cordilleran type setting can explain the widespread distribution of magmatism and gold mineralization associated with shortening as well as the required heat source triggering hydrothermal fluid flow along shear zones

tectonic setting of post collisional magmatism in the palaeoproterozoic svecofennian orogen sw finland abstract five bimodal post collisional intrusions in southwestern finland have been investigated geochemically the mafic rocks are shoshonitic monzodiorites which are highly enriched in fe p ti f lree and in incompatible trace elements the felsic rocks are garnet bearing peraluminous s type anatectic granites new data on the mafic and the felsic intrusions yielded the same u pb zircon age of 1815 ma therefore the mafic and felsic intrusions are coeval but not cogenetic narrow contact metamorphic aureoles around the mafic intrusions contain garnet orthopyroxene bearing assemblages and thermobarometry indicates an intrusion depth of at least 15 km hence there was little or no unroofing after peak regional metamorphism at 4 6 kbar the geochemical characteristics of the mafic rocks suggest that they were derived from subcontinental lithospheric mantle that was previously enriched by fluids released during svecofennian subduction it is suggested here that hot upwelling asthenosphere convectively removed subcontinental lithospheric mantle and triggered partial melting of the enriched parts of the mantle uprising mafic melts increased the already high temperatures at mid crustal levels and caused granulite facies metamorphism crustal anatexis and production of granitic melts

the effect of an equilibrated melt phase on the shear creep and attenuation behavior of polycrystalline olivine the impact of a chemically and texturally equilibrated melt phase on the shear creep and attenuation behaviors of polycrystalline olivine has been measured experimentally at seismic to subseismic frequencies the experiments were performed on aggregates that had a particularly uniform and fine grain size 3 m the effect of the melt phase 5 vol is to decrease the newtonian viscosity by a factor of 6 there is no dramatic disaggregation effect or melt induced plummeting of the shear modulus both the melt free and melt bearing aggregates display an attenuation band of the form qg 1 f 1 4 this response cannot be attributed to a variation in microstructure it is intrinsic to the diffusional creep behavior there is no unique effect of the melt phase on the attenuation response the slight increase in absorption of partial melt specimens is explained fully by the effect of the texturally equilibrated melt on aggregate viscosity

extraction of absolute p velocity from receiver functions summary receiver functions constitute an important tool in regional and global studies of crust and upper mantle structure and are frequently employed to constrain crustal thickness and bulk vp vs ratio the methods commonly used to compute these parameters assume a known crustal p velocity however this quantity trades off with crustal thickness we demonstrate that p velocity and vp vs ratio can be readily computed through solution of a linear system of equations incorporating traveltimes of direct conversions and free surface reverberations representing a range of horizontal slowness determination of crustal thickness follows trivially we apply this approach to data from station hyb on the indian craton to determine bulk vp vs ratio p velocity and thickness of 1 79 0 007 6 1 0 13 km s 1 and 30 5 0 8 km respectively the addition of average crustal p velocity to the suite of parameters accessible from receiver function data may prove useful in constraining bulk crustal composition and its secular evolution

migmatization related to mafic underplating and intra or back arc spreading above a subduction boundary in a 2 0 1 8 ga accretionary orogen sweden abstract absolute ages of migmatization and protolith formation and constraints on the timing of ductile deformation in two major lithotectonic units in the south western part of a 2 0 1 8 ga orogenic belt in the fennoscandian shield south eastern sweden have been determined using u pb ion probe analysis of different generations of zircon in veined gneisses and leucocratic granite detrital and xenocrystic zircon in paragneiss and garnet bearing leucogranite respectively in the bergslagen lithotectonic unit show ages of 2 1 2 0 ga and 1 9 ga deposition of the sedimentary material occurred during or after a subduction related magmatic event at 1 91 1 87 ga two orthogneiss protoliths formed during this magmatic event around 1 88 ga while most zircon in the leucosome in a third migmatitic orthogneiss was inherited from a 1 85 ga igneous protolith a polyphase tectonothermal evolution with anatexis under low p metamorphic conditions around 1 86 ga m1 and 1 84 1 81 ga m2 is inferred for the migmatitic gneisses in the bergslagen unit garnet bearing leucogranite crystallised around 1 84 1 83 ga close in time to major folding of the m1 gneissic fabric a previously unrecognised 1 86 1 85 ga ductile deformational event under medium grade metamorphic conditions has been identified in the adjacent lithotectonic unit to the south smaland lithotectonic unit close in time to the m1 event in the bergslagen unit to the north by constraining the timing of anatexis and comparing with information bearing on crustal thickness excess mass at depth and the character and age of magmatic activity it is inferred that anatexis in the bergslagen lithotectonic unit is related to pulses of mafic underplating during the early stages of two separate subduction related magmatic episodes after the 1 91 1 87 ga magmatic event it is suggested that each pulse was related to intra or back arc spreading above a subduction boundary which had entered a retreating mode with separate long periods 20 50 ma of extension or transtension this study challenges the need to invoke crustal thickening related to plate collision at 1 9 1 8 ga as a mechanism to explain high grade metamorphism in the southern part of the 2 0 1 8 ga orogen instead a solely accretionary tectonic model involving an overriding plate along an active continental margin with significant extensional or transtensional crustal deformation is preferred

modelling the thermal evolution of a collisional precambrian orogen high heat production migmatitic granites of southern finland abstract we present results of thermal and geochemical modelling of paleoproterozoic southern finland which is characterized by formation of high heat production migmatitic granites post orogenic granitoids and high temperature low pressure metamorphism at 1 85 1 78 ga during late svecofennian time metamorphic grade is mostly amphibolite facies 4 5 kbar 650 700 c but also granulite facies 6 kbar up to 800 c occurs our results suggest that the thermal evolution of the area can be modelled as a process of rapid crustal thickening up to 70 km in a plate collision at ca 1870 1860 ma ago followed by conductive heating of the crust simultaneous exhumation and partial melting of middle crust and emplacement of the melts in the upper crust as late orogenic high heat production granites at about 1825 ma ago the upper crust collapsed gravitationally at about 1820 ma but the crust was still thick 57 km when the main orogenic era terminated about 1800 ma ago in modelling the thermal evolution we applied crustal heat production values constrained by geochemical data and a conservative estimate i e relatively low value for mantle heat flow density the modeled temperatures at 30 50 km depths in the thickened crustal stack exceed the dehydration melting temperatures of micas at 40 km 750 c for muscovite and 800 c for biotite respectively 25 35 ma after the collision and thickening of crust the results explain the high temperature low pressure metamorphism in southern finland as an inevitable consequence of crustal thicknening leading to an increase of the total heat production of the crust crustal thicknening is the main heat source for melting and metamorphism of the orogeny advective and latent heat brought by the melts further increased the temperatures at the emplacement depth and contributed to the granulite facies metamorphism that is spatially related to the migmatitic granites geochemical models suggest that the high concentrations of u and th in the migmatitic granites can be attributed to about 20 vol partial melting of deeply buried sediments and volcanic rocks about 36 71 of the original u 18 54 of th and 34 46 of k respectively in the source rock types entered the melt phase no anomalous concentrations of these elements in the source rock are required in addition to explaining the migmatitic late orogenic granite and post orogenic granitoid genesis the collision model implies that the middle lower crust and the upper mantle continued warming until the mesoproterozoic times which suggests a causal link between the svecofennian thermal evolution and the extensive bimodal rapakivi granite and anorthosite magmatism in the fennoscandian shield about 200 250 ma later

1 88 1 87 ga post kinematic intrusions of the central finland granitoid complex a shift from c type to a type magmatism during lithospheric convergence abstract the evolution of the svecofennian orogen spans from 1 95 ga to 1 80 ga with the main crust forming activity having taken place 1 90 1 87 ga ago subsequent deformation was largely concentrated to southern finland where transpressional structures indicate lithospheric convergence until 1 80 ga much of the svecofennian orogen in finland is occupied by the central finland granitoid complex cfgc that mainly consists of i type granodiorite and granite these 1 89 1 88 ga intrusive rocks are typically foliated and considered synkinematic a distinct suite of post kinematic undeformed or slightly foliated plutons crosscut the synkinematic rocks they are predominantly quartz monzonites and monzogranites and have been divided into three types according to their petrographic mineral and chemical characteristics compared to the calc alkaline synkinematic granitoids the post kinematic suite generally has an alkaline affinity with higher fe ti k ba zr and nb and lower mg ca and sr at a given sio 2 content the type 1 post kinematic plutons are peraluminous and contained a prominent sedimentary component in their source the types 2 and 3 plutons are marginally metaluminous to peraluminous the type 2 granites especially those in the western cfgc approach the classic 1 65 1 54 ga rapakivi granites of southern finland in their petrographic characteristics magmatic association tholeiitic mafic rocks and elevated contents of incompatible elements and hence have a type characteristics the type 3 plutons contain a pyroxene bearing margin type 3a or contain pyroxene throughout type 3b the type 3b rocks have c type charnockite magma type affinities and some plutons of this subgroup are alkaline the type 3a plutons appear to be transitional between the type 2 and type 3b plutons the synkinematic magmatic episode in the cfgc area probably involved partial melting of intermediate high k rocks in the lower crust with a magmatic addition from a mafic underplate this magmatic episode left a hornblende and biotite poor granulitic residue in the lower crust heat from the mafic underplate triggered partial melting of the granulite to produce the post kinematic magmatism and also introduced mafic material into the post kinematic magmas the post kinematic magmatism took place over quite a short time period ca 15 ma during overall lithospheric convergence it extended beyond the cfgc area and registers a temporal shift from the northeast toward the west and a general change in the character of magmatism from c type to a type the post kinematic magmatism probably resulted from extensional or transtensional events modifying the tectonically thickened crust partial melting of the mafic lower crust produced c type magmatism soon after the first melting episode the crust was already rather rigid and allowed the mafic magmas to follow a tholeiitic trend when the bimodal a type magmas were formed in the western cfgc

prevalence of viscoelastic relaxation after the 2011 tohoku oki earthquake seafloor global positioning system observations immediately after the great 2011 tohoku oki earthquake provide unambiguous evidence for the dominant role of viscoelastic relaxation in short term postseismic deformation rather than just afterslip on the fault as is commonly assumed this paper presents revised estimates of the afterslip following the 2011 tohoku oki earthquake based on seafloor gps observations made immediately after the earthquake tianhaozhe sun et al obtain unambiguous evidence for the dominant role of viscoelastic relaxation in short term postseismic deformation rather than elastic afterslip on the fault as is commonly assumed they conclude that models which erroneously assume an elastic earth substantially overestimate afterslip downdip of the rupture zone and underestimate afterslip updip after a large subduction earthquake crustal deformation continues to occur with a complex pattern of evolution1 this postseismic deformation is due primarily to viscoelastic relaxation of stresses induced by the earthquake rupture and continuing slip afterslip or relocking of different parts of the fault2 3 4 5 6 when postseismic geodetic observations are used to study earth s rheology and fault behaviour it is commonly assumed that short term a few years deformation near the rupture zone is caused mainly by afterslip and that viscoelasticity is important only for longer term deformation6 7 however it is difficult to test the validity of this assumption against conventional geodetic data here we show that new seafloor gps global positioning system observations immediately after the great tohoku oki earthquake provide unambiguous evidence for the dominant role of viscoelastic relaxation in short term postseismic deformation these data reveal fast landward motion of the trench area opposing the seaward motion of gps sites on land using numerical models of transient viscoelastic mantle rheology we demonstrate that the landward motion is a consequence of relaxation of stresses induced by the asymmetric rupture of the thrust earthquake a process previously unknown because of the lack of near field observations our findings indicate that previous models assuming an elastic earth will have substantially overestimated afterslip downdip of the rupture zone and underestimated afterslip updip of the rupture zone our knowledge of fault friction based on these estimates therefore needs to be revised

timing of palaeoproterozoic intra orogenic sedimentation in the central fennoscandian shield evidence from detrital zircon in metasandstone detrital zircon u pb sims data on quartz rich metasandstone units presumed to belong to the upper part of the svecofennian stratigraphy in southeastern finland and east central sweden suggest the existence of clastic sedimentary basins between the two main orogenic phases at 1 89 1 86 ga and 1 83 1 79 ga during a period referred to as the intra orogenic phase 1 86 1 83 ga stratigraphically below the metasandstone at hamrange east central sweden is a metadacite with an u pb zircon tims age of 1888 6 ma which indicates the maximum age of sedimentation it also indicates that an earlier proposed correlation of hamrange metavolcanic rocks and 1 86ga equivalents at los to the northwest must be rejected instead there is a temporal affinity to the metavolcanic rocks in the bergslagen province to the south or southern finland to the east quartz rich metasandstone samples from four localities luukkola pyhantaka and tiirismaa in finland and hamrange in sweden yield multimodal detrital zircon age distributions with main populations at 2 95 2 60 ga 2 10 1 95 ga and 1 92 1 85 ga the groups are similar in all four samples and they are comparable to previously reported detrital ages in this part of the fennoscandian shield the oldest zircon analysed gave an age of 3 32 ga tiirismaa the maximum ages of sedimentation and of subsequent deformation and metamorphism indicated by the youngest detrital zircon from the four localities are 1842 10 ma luukkola 1865 11 ma pyhantaka 1848 13 ma tiirismaa and 1855 10 ma hamrange respectively possible source rocks for these zircon grains are found within and around the vast ljusdal batholith in sweden and in the arc complexes of western and southern finland it is concluded that several intra orogenic sedimentary basins existed during the time interval 1 86 1 83 ga ago between two major orogenic events in the fermoscandian shield less

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

three dimensional field perspective on deformation flow and growth of the lower continental crust dharwar craton india 1 the study of fabric development and juvenile batholith emplacement across the tilted crustal section of the eastern dharwar craton shows that horizontal constrictional deformation affected large volumes of the midcrust and lower crust at the time of regional partial melting and magmatic accretion constriction is achieved by a combination of coeval shallow and steep planar fabrics sharing a common horizontal elongation direction two sets of conjugate strike slip shears and extensional shear zones the eastern dharwar craton illustrates an end member deformation mode by which a particularly weakened lithosphere responds to shortening by developing distributed horizontal plane strain on a crustal scale resulting from the combination of crustal shortening and lateral gravity driven flow thinning accompanying constrictional deformation is interpreted as compensating for juvenile magmatic accretion and thickening of greenstone belts and as acting to maintain a stable crustal thickness such a midcrustal to lower crustal deformation process may provide a resolution of the batholithic room problem in a softened crust submitted to lateral shortening and may explain nearly isobaric retrograde pressure temperature time paths of high temperature low pressure high grade terrains

along arc variation in the 3 d thermal structure around the junction between the japan and kurile arcs the thermal structure in subduction zones has a strong influence on seismogenesis and arc volcanism traditional 2 d models have been used to provide reasonable agreement between models and observations but in a number of cases clear 3 d effects are present one such case is in the northern japan subduction system at the junction between japan and kurile arcs surface heat flow and the occurrence of intermediate depth seismicity are different than in the tohoku and hokkaido regions we investigate the effects of 3 d slab geometry and a local deepening of slab mantle decoupling depth on the thermal structure in this region based on 3 d finite element approach we find that both effects produce the along arc variation of slab surface temperature which could reach 100 c the warmer region arises through 3 d effects of thermal conduction and the colder region arises through localized slow incoming flow in the case where 3 d slab geometry is taken into account 3 d flow arises where a local deepening of slab mantle decoupling depth is assumed which leads to both warmer and colder regions the effects on surface heat flow are small while intermediate depth seismicity in the subducted crust is suggested to be controlled by temperature dependent phase transitions the predicted changes in thermal structure are not sufficient to cause the observed deepening of seismicity this suggests that the thermal structure of this subduction zone may be more strongly influenced by time dependent deformation of the overriding crust and slab

u pb zircon ages and rb sr whole rock isotope studies of early proterozoic volcanic and plutonic rocks near tampere southern finland abstract the tampere schist belt southern finland is a well preserved volcanic sedimentary belt in the early proterozoic svecofennian terrain the metasedimentary rocks of the belt are dominated by turbiditic greywackes siltstones and mudstones the metavolcanics range from basalt to rhyolite in composition intermediate rocks being the most common near tampere the belt comprises an east west trending major syncline the u pb zircon ages of the 1 9 ga old metavolcanics cover an age interval of 15 ma the oldest age 1904 4 ma is from the intermediate unit at orivesi which is dominated by calc alkaline high k dacites and andesites this unit is the lowest in the northern limb of the syncline at orivesi at ylojarvi a sample from the lower volcanic unit has an age of 1898 4 ma while a sample from the upper volcanic unit has the youngest crystallisation age of 1889 5 ma these ages agree with the structure and stratigraphy of the belt the implications of the 1891 16 and 1889 19 ma ages of two feldspar porphyries remain obscure here the error limits are wide and it is not known with certainty whether the rocks are dykes sills or volcanics an age of 1901 28 ma was obtained for the varmala granitoid which intrudes the schists although the error is large it is possible that some of the zircon has been inherited from older rocks the rb sr whole rock ages of the volcanics agree within error limits with the u pb zircon ages the rb sr age of the hameenkyro batholith is however about 100 ma lower than the equivalent u pb age the i sr values of the metavolcanics and the 1 882 ga old hameenkyro batholith range between 0 7020 and 0 7033 these low figures suggest that their protoliths separated from upper mantle sources within a short time interval the sm nd and lu hf isotopic data available in the literature support the conclusion that the period around 1 9 ga represents an important episode of continental growth which is particularly evident in the svecofennian ketilidian trans hudsonian terrains

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

crust and mantle structure across the basin and range colorado plateau boundary at 37 n latitude and implications for cenozoic extensional mechanism we present new evidence on the seismic velocity and density of the crust and upper mantle along a 200 km long transect across the eastern basin and range and western colorado plateau at 37 n latitude receiver functions computed from the p waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations the crust is 30 35 km thick within the eastern basin and range and increases over a distance of 100 km at the western edge of the colorado plateau reaching a maximum of approximately 45 km east of the hurricane fault the timing of crustal multiples within the reciever functions were used to estimate the v p v s of the crust along the profile and we found that the western colorado plateau crust is characterized by a high poisson s ratio 0 28 0 29 indicative of a crust with an average mafic composition we estimated the upper mantle lid thickness along our profile based on teleseismic p wave travel times and constraints provided by gravity data our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the basin and range to 100 km beneath the western colorado plateau although the basin and range lithosphere may have significant thickness variations the thick strong mafic crust and thicker mantle lid under the colorado plateau can account for the relative geologic stability and subdued magmatism of the plateau during laramide compression and cenozoic extension compared to surrounding regions the crustal and lithospheric thinning across the tectonic boundary occurs over a short distance 100 km suggesting it is a geologically young feature produced by a predominantly mechanical response to late cenozoic extension our new lithosphere model at 37 n latitude is consistent with the existence in early cenozoic time of a flat subducted slab at 100 km depth and a relict sevier laramide 50 60 km thick crustal welt and 60 100 pure shear extension values of 1 6 2 0 during the late cenozoic

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

timing of accretion intra orogenic sedimentation and basin inversion in the paleoproterozoic svecofennian orogen the pyhantaka area southern finland abstract the pyhantaka area in southern finland represents the accretionary svecofennian orogen presently exhumed to upper mid crustal level the post 1 87 ga volcanic sedimentary pyhantaka formation is separated from underlying cordierite paragneiss metagraywacke by an unconformity marked by an ancient weathering surface on top of the paragneiss and ultramature quartzite at the base of the pyhantaka formation structural studies and u pb datings carried out in the paragneisses metavolcanic rock interlayer and crosscutting granitoid rocks revealed four tectonic units separated from each other by shear zones and representing different depositional ages pre 1 87 ga and post 1 87 ga yet unidentified post 1 87 ga rock sequences are anticipated in the svecofennian orogen the model of tectonothermal evolution in the pyhantaka area includes contraction at 1 89 ga causing rapid burial of graywackes harkala graywackes to mid crustal levels where they were metamorphosed into cordierite gneisses and intruded by syntectonic granite at 1875 ma deposition of exhumation related graywackes himattu graywackes after 1 87 ga followed by development of a weathering surface and deposition of quartz sands indicate gradual stabilization of the upper crust a sequence of quartz sands immature sands volcaniclastic sands and basaltic rocks the pyhantaka formation and graywackes on top kesua graywackes were deposited in an intra orogenic rift basin during crustal extension that may have initiated 1 85 ga ago contraction at 1825 ma caused basin inversion with thrusting and folding of the post 1 87 ga sequence all rocks in the pyhantaka area were subject to a new high grade metamorphic event that culminated at 1 82 ga subsequent uplift of a pre 1 87 ga rock sequence relative to the post 1 87 ga sequence resulted in the present pattern of tectonic units the inferred evolution at pyhantaka conforms to several tectonic models presented for the svecofennian orogen however the occurrence of an ancient weathering surface and ultramature quartzite an evidence for a 10 20 ma stable period is more compatible with continental interior setting with outboard subduction than active continental margin setting during deposition of the pyhantaka formation

structural setting of late svecofennian granites and pegmatites in uusimaa belt sw finland age constraints and implications for crustal evolution complex late svecofennian deformation patterns and voluminous granitic magmatism at 1 85 1 80 ga characterise the palaeoproterozoic uusimaa belt in sw finland detailed structural analysis and ion

lithospheric and upper mantle structure of the indian shield from teleseismic receiver functions receiver functions derived from teleseismic p waveforms of 297 earthquakes recorded at broadband station hyderabad hyb are analysed to investigate the lithospheric and upper mantle structure in the vicinity of the station located in the northeastern part of the archaean dharwar craton the internal structure of the 33 2 km thick crust is simple it can be approximated by a single layer with s velocity linearly increasing with depth from 3 4 to 4 0 km s the average being 3 7 0 2 km s the crustal poisson s ratio is 0 25 0 01 a seismic discontinuity at 90 km depth is inferred from a signal particularly prominent on the sh component and possibly indicative of a layer of depth localized anisotropy in the lithospheric mantle the two principal mantle discontinuities are at depths of 406 and 659 km which are close to the global averages

seismic tomography and earthquake locations in the nicaraguan and costa rican upper mantle the central american subduction zone exhibits large variations in geochemistry downgoing plate roughness and dip and volcano locations over a short distance along the arc results from joint inversions for vp vp vs and hypocenters from the tomography under costa rica and nicaragua tucan experiment give insight into its geometry and structure in both costa rica and nicaragua the intermediate depth seismic zone is a single layer no more than 10 to 20 km thick tomographic images show that throughout nicaragua and costa rica the slowest mantle p wave velocities appear below and behind the volcanic front indicating likely zones of highest temperature extending 80 to 120 km depth a sheet of high vp vs thought to be caused by melt is imaged directly beneath the nicaraguan volcanoes whereas a weaker broader anomaly is imaged beneath the costa rican volcanoes potentially indicating a greater extent of melting beneath nicaragua within the downgoing plate anomalously low velocities occur at least 20 30 km below wadati benioff zone seismicity to depths of 140 km beneath nicaragua and to 60 km depth beneath costa rica they indicate 10 20 serpentinized upper mantle of the downgoing plate beneath nicaragua similar to that inferred from refraction seaward of the trench but continuing to subarc depths this unusually hydrated lithosphere may introduce more water into the nicaraguan mantle initiating increased amount of melting and fluid flux to the arc

svecofennian detrital zircon ages implications for the precambrian evolution of the baltic shield metasediments intruded by 1 90 1 87 ga old plutonic rocks form the oldest major proterozoic crustal component in the svecofennian domain of the baltic fennoscandian shield their ndtdm model ages and conventional multigrain zircon u pb ages between 2 4 and 2 1 ga have previously been interpreted either as mixing ages between 1 9 ga old juvenile materials and a minor archaean component or as actual rock and protolith ages to resolve the ensuing controversy 120 individual detrital zircons from svecofennian metasediments in sweden and finland were analysed using the shrimp ion microprobe the oldest materials in this array are a 3 44 ga old zircon from the tampere schist belt in finland and a 3 32 ga old crystal from southeastern sweden about 30 of the analysed crystals are 2 97 2 60 ga old while 65 have ages between 2 12 and 1 88 ga thus there is no evidence of 2 6 2 1 ga old protoliths but the age range of the proterozoic zircons indicates that a major area of 2 1 1 9 ga old crust was in erosional position 1 9 ga ago this implies that the formation of palaeoproterozoic crust in the baltic shield or its one time close neighbourhood must have commenced 100 200 ma earlier than hitherto assumed in conjunction with previously obtained isotopic data the youngest detritus ages of the present study constrain the age of svecofennian sedimentation it can also be concluded that the archaean zircons found in quartzites from southern sweden may have been derived from source areas to the southwest of the central svecofennian marine depositional basin the so called bothnian basin separating southern sweden from the archaean craton in the northeastern part of the shield

sharp thermal transition in the forearc mantle wedge as a consequence of nonlinear mantle wedge flow 1 in the forearc mantle wedge the thermal field depends strongly on slab driven mantle wedge flow the flow is in turn affected by the thermal field via the temperature dependence of mantle rheology using thermal modeling we show that the nonlinear feedback between the thermal and flow fields always leads to complete stagnation of the mantle wedge over a shallow weakened part of the slab mantle interface and an abrupt onset of mantle flow further down dip the abrupt increase in flow velocity leads to a sharp thermal transition from a cold stagnant to a hot flowing part of the wedge this sharp thermal transition is inherent to all subduction zones explaining a commonly observed sharp arc ward increase in seismic attenuation

continental margin magmatism and migmatisation in the west central fennoscandian shield the ljusdal batholith ljb is a major component of the central svecofennian domain in sweden it is separated from the bothnian basin to the north by the 1 82 1 80 cia crustal scale hassela shear zone hsz the ljb has emplacement ages of 1 86 1 84 cia is mainly alkali calcic metaluminous has epsilon nd values between 0 3 and 1 2 and was formed in a magmatic arc setting during the svecokarelian orogeny the ljb was affected by at least three fold episodes large scale folded screens of migmatised metasedimentary rocks occur in the eastern part of the batholith and to the north of the hsz there is a 50 km wide diatexite belt the transition belt trb consisting of 1 88 1 85 ga granitoids is located at the northwestern extension of this belt a calc alkaline and peraluminous composition combined with negative epsilon nd values 1 7 to 0 8 indicates a large proportion of metasediments in the source for these granitoids u pb sims data on zircon rims from migmatites and leucogranites to the north and east of ljb yield ages of 1 87 1 86 ga i e coeval with the granitoids of the ljb and the trb there is thus a close relationship between the ljb the trb and the migmatites in both space and time syn migmatitic shearing along the hsz indicates that a proto hsz was initiated already at c 1 86 ga and the location of the proto hsz is inferred to be controlled by two older nuclei present in the lower parts of the crust as crustal scale shear zone systems are known to act as ascent pathways for sheet like flow in active orogenies the trb may represents accumulations of melts that were attracted and extracted by the proto hsz and intruded in a block that was not pervasively affected by subsequent shear along the hsz an active continental margin setting for the ljb implies subduction at c 1 86 ga and provides a heat source for both the migmatites and the trb a later migmatisation at 1 82 ga has been recorded to the south of the hsz within the ljb the 1 82 ga stromatic migmatites are folded by f 2 folds and the fabric is truncated by 1 80 cia pegmatites

timing of magmatism and migmatization in the 2 0 1 8 ga accretionary svecokarelian orogen south central sweden the palaeoproterozoic 2 0 1 8 ga svecokarelian orogen in central sweden consists of a low pressure predominantly medium grade metamorphic domain central part of bergslagen lithotectonic unit enclosed to the north and south by low pressure migmatite belts two periods of metamorphism 1 87 1 85 and 1 83 1 79 ga are known in the migmatite belts in this study new u th pb ion microprobe data on zircon and monazite from twelve samples of locally migmatized gneisses and felsic intrusive bodies determine both protolith and metamorphic ages in four sample areas north of stockholm inside or immediately adjacent to the medium grade metamorphic domain two orthogneiss samples from the rimbo area yield unusually old protolith ages of 1909 4 and 1908 4 ma while three orthogneisses from the skutskar and forsmark areas yield more typical protolith ages between 1901 3 and 1888 3 ma migmatized paragneiss samples from this and two earlier studies contain a significant detrital component sourced from this 1 9 ga magmatic suite they are interpreted to be deposited contemporaneously with or shortly after this magmatism migmatization of the paragneiss at rimbo was followed by intrusion of leucogranite at 1846 3 ma even in the other sample areas to the north hedesunda tierp skutskar and forsmark metamorphism including migmatization is constrained to the 1 87 1 85 ga interval and penetrative ductile deformation is limited by earlier studies in the forsmark area to 1 87 1 86 ga however apart from a metamorphic monazite age of 1863 1 ma precise ages were not possible to obtain due to the presence of only partially reset recrystallized domains in zircon or highly discordant u rich metamict and altered metamorphic rims migmatization was contemporaneous with magmatic activity at 1 87 1 84 ga in the bergslagen lithotectonic unit involving a mantle derived component and there is a spatial connection between migmatization and this magmatic phase in the hedesunda tierp sample area the close spatial and temporal interplay between ductile deformation magmatism and migmatization the p t metamorphic conditions and the continuation of similar magmatic activity around and after 1 8 ga support solely accretionary rather than combined accretionary and collisional orogenic processes as an explanation for the metamorphism the generally lower metamorphic grade and restricted influence of the younger metamorphic episode at least at the ground surface level distinguishes the central part of the bergslagen lithotectonic unit from the migmatite belts further north and south

three dimensional attenuation structure beneath the northeastern japan arc estimated from spectra of small earthquakes abstract a fine scale three dimensional attenuation structure beneath the northeastern japan arc has been obtained using a joint inversion for source parameters site response and q values data used for this study are p wave spectra of microearthquakes occurring in the subducting pacific plate the subducting plate is found to have higher q values than the overlying mantle there is an along arc variation in the distribution of low q zones in the crust and the mantle wedge within the study area for the southern part of the study area low q zones are distributed in the crust beneath the volcanic front they extend into the mantle wedge and become deep toward the west that is toward the backarc region on the other hand the northern part of the study area has low q zones at two depth ranges at shallower depths just beneath the volcanic front and at the deeper part of the mantle wedge beneath the backarc region the region without active volcanoes along the volcanic front has relatively high mantle q values seismic activity in the lower plane of the double planed deep seismic zone is high beneath this relatively high q region of the mantle wedge spatial distribution of low q zones found in the present study is nearly consistent with that of the low velocity zones estimated from studies of travel times tomography partial melting zones in the mantle wedge whose existence has been suggested from the study of seismic velocity structure and laboratory experiments are located in the low q zones obtained in the present study these results suggest that spatial distribution of q values is closely related to heterogeneous distribution of temperature in the mantle wedge

preservation of palaeoproterozoic early svecofennian structures in the orijarvi area sw finland evidence for polyphase strain partitioning abstract the predominantly migmatitic palaeoproterozoic uusimaa belt preserves early lower grade svecofennian structures in the orijarvi area in sw finland this study aims at explaining the deformational history responsible for its preservation and also at defining the age of the early svecofennian deformation detailed structural analysis reveals that the preservation was enabled by polyphase strain partitioning which initiated during the early svecofennian d2 deformation 1875 ma ago as revealed by ion microprobe u pb data on zircons from granodioritic and intermediate syn d2 intrusive dykes the d2 structures were low strain upright folds at high crustal levels and sub horizontal high strain folds at deeper crustal levels the sub horizontal d2 structures were refolded into upright folds during the subsequent late svecofennian d3 deformation whereas the upright d2 structures behaved as almost rigid blocks that caused strain partitioning into high strain zones along the block margins this accounts for the low cumulative strain in specific parts of the orijarvi area further strain partitioning during d4 caused reverse dip slip movements along regional scale shear zones crustal depth controlled the metamorphic grade during d2 when local migmatisation took place at deep crustal levels later metamorphic overprint during d3 deformation is evident from post d2 growth of sillimanite and a second generation of andalusite similarities in the structural patterns between the orijarvi area and the tampere vammala area 100 km to the north suggest that irrespective of the age of the later overprint subsequent deformation was localised along the margins of the early formed upright domains while the low grade rocks within the domains were preserved

an early proterozoic ophiolite the jormua mafic ultramafic complex northeastern finland abstract the 1 96 ga old jormua complex comprises a fault bounded unit 2 5 20 km in the centre of the kainuu schist belt at the eastern margin of the early proterozoic svecokarelides the complex was dismembered and metamorphosed to lower amphibolite facies during the svecokarelian orogeny in the intact complex the basal unit was formed by serpentinites the middle unit of mafic dykes and the uppermost extrusive unit of basaltic pillow lavas gabbroic rocks are present as intrusive bodies in the serpentinites and narrow interdyke screens within the dyke unit trondhjemitic rocks occur as minor segregations in the gabbroic rocks or as cross cutting narrow dykes in the mafic dyke unit the dyke unit is locally composed exclusively of subparallel metabasalt and metadolerite dykes in places interdyke metagabbro and serpentinite screens are abundant well preserved chilled margins are common and many dykes are split and intruded by younger dykes the chemical composition of the serpentinites indicates derivation from harzburgitic dunitic mantle peridotites the gabbroic rocks which represent obvious tholeiitic cumulates show extreme differentiation similar to gabbros of the high ti ophiolites and gabbros dredged from present midocean ridges in chemistry the trondhjemitic rocks are close to granitic rocks from mid ocean ridges and high ti ophiolites the similar chemical composition of the dykes and lavas of the complex indicates that they are cogenetic both being tholeiitic metabasalts with chondrite normalized ree patterns ranging from slightly lree depleted to slightly lree enriched ones compared with modern rocks the jormua metabasalts most closely resemble transitional midocean ridge basalts it is suggested that the jormua complex and related mafic ultramafic bodies in eastern finland originated as divergent margin ophiolites concomitantly with the break up of the archaean craton in the extensional stage of the svecokarelian orogeny which culminated in the opening of a major ocean after 1 97 ga the tectonic emplacement of these ophiolites into their present locations occurred in connection with thrusting during the compressional stage of the orogeny about 1 9 ga ago

high pressure creep of serpentine interseismic deformation and initiation of subduction the supposed low viscosity of serpentine may strongly influence subduction zone dynamics at all time scales but until now its role could not be quantified because measurements relevant to intermediate depth settings were lacking deformation experiments on the serpentine antigorite at high pressures and temperatures 1 to 4 gigapascals 200 to 500 c showed that the viscosity of serpentine is much lower than that of the major mantle forming minerals regardless of the temperature low viscosity serpentinized mantle at the slab surface can localize deformation impede stress buildup and limit the downdip propagation of large earthquakes at subduction zones antigorite enables viscous relaxation with characteristic times comparable to those of long term postseismic deformations after large earthquakes and slow earthquakes antigorite viscosity is sufficiently low to make serpentinized faults in the oceanic lithosphere a site for subduction initiation

p and s velocity tomography of the mariana subduction system from a combined land sea seismic deployment seismic imaging provides an opportunity to constrain mantle wedge processes associated with subduction volatile transport arc volcanism and back arc spreading we investigate the seismic velocity structure of the upper mantle across the central mariana subduction system using data from the 2003 2004 mariana subduction factory imaging experiment an 11 month deployment consisting of 20 broadband seismic stations installed on islands and 58 semibroadband ocean bottom seismographs we determine the three dimensional vp and vp vs structure using over 25 000 local and over 2000 teleseismic arrival times the mantle wedge is characterized by slow velocity and high vp vs beneath the fore arc an inclined zone of slow velocity underlying the volcanic front and a strong region of slow velocity beneath the back arc spreading center the slow velocities are strongest at depths of 20 30 km in the fore arc 60 70 km beneath the volcanic arc and 20 30 km beneath the spreading center the fore arc slow velocity anomalies occur beneath big blue seamount and are interpreted as resulting from mantle serpentinization the depths of the maximum velocity anomalies beneath the arc and back arc are nearly identical to previous estimates of the final equilibrium depths of mantle melts from thermobarometry strongly indicating that the low velocity zones delineate regions of melt production in the mantle the arc and back arc melt production regions are well separated at shallow depths but may be connected at depths greater than 80 km

the thermotectonic evolution of a proterozoic low pressure granulite dome west uusimaa sw finland three fold generations have been recognized in svecofennian rocks 1 800 ma from west uusimaa sw finland the first one f1 might be related to thrusting and imbrication tectonics at plate collision contacts the main generation f2 is due to a n s horizontal crustal shortening which created at first e w trending upright folds in the whole region and later tightened these f2 folds in the western part of the belt whereas conjugate shear zones and tectonic lenses of competent rock bodies developed in the eastern part simultaneously the metamorphic conditions rose from amphibolite to granulite facies in this eastern part which is known as the west uusimaa complex the amphibolite to granulite facies transition zone along the western boundary of the granulite facies complex is studied in detail a number of prograde mineral reactions are telescoped in this transition zone the breakdown of biotite and amphibole to ortho clino pyroxene in metaigneous rocks the appearance of garnet in cordierite bearing metapelites and the appearance of scapolite in calcareous rocks distinct mineralogical changes also occur in this zone which cross cuts all major structures and rock units and are only affected by late f3 folding open disharmonic folds with approximately n s trending axial planes and young shear zones associated with pseudotachylite generation the absence of any evidence of block faulting and tilting of the crust that could be associated with the granulite complex suggests that the whole region represents one crustal level

deformation of antigorite serpentinite at high temperature and pressure abstract deformation experiments were conducted in a griggs type apparatus to constrain the rheology of antigorite serpentinite at high temperatures and pressures samples deformed at temperatures of 400 625 c and pressures of 0 85 1 5 gpa experienced semi brittle deformation resulting in localization on faults at 5 to 25 strain samples deformed at 300 c exhibited transitional behavior the stresses measured for our samples are in good agreement with recent experiments conducted in the deformation dia apparatus however our results indicate that steady state ductile flow is not achieved in contrast with interpretations based on the deformation dia experiments extrapolation of our data to geologic conditions suggests that serpentinites may be weak enough during localized frictional sliding to account for slab wedge decoupling inferred from geologic observations experiments conducted above the thermal stability of antigorite at 700 c and 1 5 gpa show evidence for distributed deformation this observation does not support the hypothesis that intermediate depth earthquakes result directly from dehydration embrittlement in addition samples exhibit velocity strengthening from 300 to 700 c a frictional behavior that inhibits earthquake nucleation

three dimensional attenuation structure of the hikurangi subduction zone in the central north island new zealand summary we use waveform data from 2708 earthquakes recorded by dense seismograph deployments in the central north island of new zealand to image the 3 d attenuation structure of the hikurangi subduction zone down to ca 300 km depth attenuation images are obtained by determining the quality factor of p waves qp using a t inversion with a previously determined 3 d seismic velocity model we have included limited frequency dependence for qp with qp being frequency independent above 10 hz and having a frequency dependence of f 10 0 5 below 10 hz the qp images provide further constraint on the large scale features associated with subduction and magmatism beneath the central north island and serve to refine interpretations of crust and upper mantle structure from vp and vp vs the subducted plate is the most prominent feature in the qp images its high qp 900 1200 is consistent with the ca 120 myr old slab being relatively cold qp is better at resolving the slab than vp the coincidence of a strong gradient in qp with the upper plane of the dipping seismic zone indicates that even below 75 km depth the upper envelope of seismicity provides a good estimate of the location of the plate interface the mantle wedge is generally imaged as a relatively low qp 400 feature below 50 km depth however there are significant changes evident in the wedge along the strike of the subduction zone the most pronounced low qp in the mantle wedge occurs from ca 50 to 85 km depth beneath the productive rhyolite dominated central segment of the taupo volcanic zone suggesting a close link between volcanism and low qp in the shallow mantle wedge there is a strong correlation between low qp l owvp and high vp vs in this part of the mantle wedge suggesting that high temperature is the controlling influence on qp there within 30 km of the surface of the slab there is a large change in qp but only a modest change in vp our results are consistent with a fluid rich viscous blanket being entrained with the motion of the subducted slab down to ca 140 km depth where hydrous melting of mantle peridotite initiates trenchward of the volcanic front a strong horizontal gradient to higher qp is imaged in the mantle wedge we interpret this as a stagnant mantle nose not involved in the corner flow of the adjacent mantle the distribution of qp within both the slab and the overlying plate provides support for dehydration embrittlement being important in promoting seismicity in the mantle of the slab where qp is high at shallow depth in the forearc it is usually underlain by low qp near the plate interface this suggests that competent terranes in the forearc may act as aquicludes increasing fluid content in the rocks below and hence might control the distribution of coupling at the plate interface

the deep crustal structure of the warakurna lip and insights on proterozoic lip processes and mineralisation abstract understanding how large igneous provinces lips formed in the proterozoic is subject to several challenges including incomplete geological exposure deformations from subsequent reworking and the lack of strong geophysical signals from the associated thermal perturbations here we apply a seismically constrained gravity inversion method to the warakurna lip region in central western australia focusing on imaging the deep crust of this region and defining the extent and intensity of mafic magmatism in particular a thick mafic underplate has been imaged in some continent scale studies but its regional extent is not well defined the results of 3d gravity inversions demonstrate an extensive area with very thick crust 45 km composed of high density materials 2830 kg m3 which is interpreted to represent a mafic underplate variations in mass excess i e underplate thickness and or density suggest that the intensity of mafic magmatism closely followed the likely lithospheric architecture of the australian continent at the time of emplacement with magmatic centres concentrated adjacent to craton margins and along translithospheric shear zones this suggests that melting was focused in the proterozoic regions with thinner lithosphere or that upwelling mantle has been diverted into this region by the roots of the archean cratons or both the mass excess is greatest beneath the magmatic centre of the lip in the west musgrave province wmp this arrangement is reflected in ni cu pge prospectivity with known deposits focused in the wmp which has a thick dense underplate in contrast the capricorn orogen and yilgarn craton which are not underplated do not possess known deposits despite extensive sills in the upper crust our results show that this method for mapping the extent and mass excess of lower crustal magmatic products provides an effective indicator of the extent and intensity of magmatism during lip events and that this can help understand lip processes including ore deposit formation

implications of grain size evolution on the seismic structure of the oceanic upper mantle abstract we construct a 1 d steady state channel flow model for grain size evolution in the oceanic upper mantle using a composite diffusion dislocation creep rheology grain size evolution is calculated assuming that grain size is controlled by a competition between dynamic recrystallization and grain growth applying this grain size evolution model to the oceanic upper mantle we calculate grain size as a function of depth seafloor age and mantle water content the resulting grain size structure is used to predict shear wave velocity v s and seismic quality factor q for a plate age of 60 myr and an olivine water content of 1000 h 10 6 si we find that grain size reaches a minimum of 15 mm at 150 km depth and then increases to 20 30 mm at a depth of 400 km this grain size structure produces a good fit to the low seismic shear wave velocity zone lvz in oceanic upper mantle observed by surface wave studies assuming that the influence of hydrogen on anelastic behavior is similar to that observed for steady state creep further it predicts a viscosity of 10 19 pa s at 150 km depth and dislocation creep to be the dominant deformation mechanism throughout the oceanic upper mantle consistent with geophysical observations we predict larger grain sizes than proposed in recent studies in which the lvz was explained by a dry mantle and a minimum grain size of 1 mm however we show that for a 1 mm grain size diffusion creep is the dominant deformation mechanism above 100 200 km depth inconsistent with abundant observations of seismic anisotropy from surface wave studies we therefore conclude that a combination of grain size evolution and a hydrated upper mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of the oceanic upper mantle our results also suggest that melt extraction from the mantle will be significantly more efficient than predicted in previous modeling studies that assumed grain sizes of 1 mm

depth of the decoupling plate interface and thermal structure under arcs subduction proceeds as thrust movements at the boundary between the subducting plate and the rigid overlying plate at depth below this mechanically decoupling plate interface the subducting plate couples with the overlying mantle and flow is induced in the mantle wedge this induced flow is the main factor that controls thermal structure under arcs depth of this decoupling to coupling transition at the plate interface is estimated from observed surface heat flow in the northeastern japan arc using a two dimensional numerical simulation of thermal structure the estimated depth is about 70 km the termination depth of thrust earthquakes agrees well with the depth of the coupling transition which indicates that these earthquakes occur at the decoupling plate interface the mechanism of this decoupling plate interface is discussed in this paper it is believed that water associated with the subducting oceanic crust reduces the frictional stress at the plate interface and thrust movement thus becomes possible petrological studies show that dehydration reaction in the subducting oceanic crust terminate at about 2 0 gpa and have little temperature dependence termination depths of thrust earthquakes estimated from seismic studies are nearly the same 60 80 km for various arcs this indicates that depth of the decoupling to coupling transition is controlled by the termination depth of dehydration reaction in the subducting oceanic crust across arc heat flow profiles observed in various subduction zones show similar features heat flow is very low in the forearc region and increases steeply at the volcanic front considering that the depth of the slab is about 100 km at the volcanic front for various arcs this heat flow feature is closely related to the depth of the slab thus it is concluded that thermal structure under arcs is mainly controlled by the depth of the pressure dependent dehydration reactions in the subducting oceanic crust

structural characteristics off miyagi forearc region the japan trench seismogenic zone deduced from a wide angle reflection and refraction study abstract the japan trench subduction zone located east of ne japan has regional variation in seismicity many large earthquakes occurred in the northern part of japan trench but few in the southern part off miyagi region is in the middle of the japan trench where the large earthquakes m 7 with thrust mechanisms have occurred at an interval of about 40 years in two parts inner trench slope and near land a seismic experiment using 36 ocean bottom seismographs obs and a 12 000 cu in airgun array was conducted to determine a detailed 2d velocity structure in the forearc region off miyagi the depth to the moho is 21 km at 115 km from the trench axis and becomes progressively deeper landward the p wave velocity of the mantle wedge is 7 9 8 1 km s which is typical velocity for uppermost mantle without large serpentinization the dip angle of oceanic crust is increased from 5 6 near the trench axis to 23 150 km landward from the trench axis the p wave velocity of the oceanic uppermost mantle is as small as 7 7 km s this low velocity oceanic mantle seems to be caused by not a lateral anisotropy but some subduction process by comparison with the seismicity off miyagi the subduction zone can be divided into four parts 1 seaward of the trench axis the seismicity is low and normal fault type earthquakes occur associated with the destruction of oceanic lithosphere 2 beneath the deformed zone landward of the trench axis the plate boundary is characterized as a stable sliding fault plain in case of earthquakes this zone may be tsunamigenic 3 below forearc crust where p wave velocity is almost 6 km s and larger this zone is the seismogenic zone below inner trench slope which is a plate boundary between the forearc and oceanic crusts 4 below mantle wedge the rupture zones of thrust large earthquakes near land e g 1978 off miyagi earthquake are located beneath the mantle wedge the depth of the rupture zones is 30 50 km below sea level from the comparison the rupture zones of large earthquakes off miyagi are limited in two parts plate boundary between the forearc and oceanic crusts and below mantle wedge this limitation is a rare case for subduction zone although the seismogenic process beneath the mantle wedge is not fully clarified our observation suggests the two possibilities earthquake generation at the plate boundary overridden by the mantle wedge without serpentinization or that in the subducting slab

seismic anisotropy and heterogeneity in the alaska subduction zone summary we determined p and s wave tomography and p wave anisotropic structure of the alaska subduction zone using 259 283 p and 73 817 s wave arrival times from 7268 local shallow and intermediate depth earthquakes recorded by more than 400 seismic stations the results show strong velocity heterogeneities in the crust and upper mantle low velocity anomalies are revealed in the mantle wedge with significant along arc variations under the active volcanoes in the mantle wedge the low velocity zone extends down to 100 150 km depth under the backarc the results indicate that h2o and fluids brought downwards by the subducting pacific slab are released to the mantle wedge by dehydration and they are subsequently transported to the surface by the upwelling flow in the mantle wedge significant p wave anisotropic anomalies are revealed under alaska the predominant fast velocity direction fvd is trench parallel in the shallow part of the mantle wedge 90 km depth and in the subslab mantle whereas the fvd is trench normal within the subducting pacific slab the trench parallel fvds in the mantle wedge and subslab mantle may be caused by 3 d mantle flow that is induced by the complex geometry and strong curvature of the pacific slab under alaska the flat and oblique subduction of the pacific slab may play a key role in forming the trench parallel fvd under the slab the trench normal fvd in the subducting pacific slab may reflect the original fossil anisotropy when the pacific plate was produced at the mid ocean ridge

timing of deformation phases within the south finland shear zone sw finland abstract in undetermined the palaeoproterozoic svecofennian crust in southern and central fennoscandia was established about 1 8 ga ago after a prolonged history of accretion and intrusion during late stages of the svecofennian orogeny deformation was partitioned into several crustal scale shear zones in present day finland sweden and estonia one such major ductile deformation zone the south finland shear zone sfsz extends for almost 200 km through the a over circle land archipelago in southwestern finland and further along the southern and southwestern coast of finland this more than a kilometer wide transpressional zone appears to have been repeatedly reactivated the deformation started with a period of regional ductile dextral shearing of igneous rocks producing striped granodioritic and tonalitic gneisses the ductile phases are locally overprinted and followed by ductile to semi ductile deformation evidenced by mylonite zones of variable width the last stage of tectonic activity along the shear zone is recorded by pseudotachylites within this study we dated zircons sims u pb and titanites id tims u pb from eight rock samples and two pseudotachylite whole rock samples ar 40 ar 39 in order to reconstruct the deformation and re activation history of the shear zone the results suggest that the medium grained gneisses underwent three distinct deformation phases separated by time intervals without regional deformation the ductile deformation within the study area initiated at similar to 1 85 ga a second more intensive deformation phase existed around 1 83 ga by which the shear zone was already well developed finally the last ductile event is recorded by similar to 1 79 ga metamorphic titanites in relatively granoblastic granitoid gneisses that nevertheless already display protomylonitic textures suggesting the initiation of large scale mylonitisation around or soon after this time the age of a pseudotachylite sample and hence the brittle deformation is bracketed between 1 78 and 1 58 ga based on the age of pegmatites cut by pseudotachylites as well as 40ar 39ar minimum ages for the pseudotachylite respectively the data imply that the rocks within the study area entered the ductile brittle transition zone due to rapid cooling and exhumation of the crust after similar to 1 79 ga less

defect microdynamics in minerals and solid state mechanisms of seismic wave attenuation and velocity dispersion in the mantle the propagation of seismic waves in the earth s mantle can be significantly affected by relaxation processes causing attenuation and velocity dispersion reduction this paper reviews the solid state mechanisms of relaxation processes based on the theory of defect microdynamics in solids together with some experimental observations on defects in minerals particularly in olivine for a given mechanism to have a significant effect on seismic wave propagation both the density and the mobility of the defects must be in an appropriate range the examination of the densities and geometry and mobilities of defects in olivine shows that dislocation and or grain boundary mechanisms can have a significant effect on seismic wave propagation although wide distributions of geometrical factors such as spacing of pinning points and of mobilities are required to explain all available data point defect mechanisms however are unlikely to be important because their densities are too small and or their mobilities are too large since the dislocation density and or grain size are determined in most cases by the long term tectonic stress seismic wave attenuation and velocity dispersion reduction involving these defects are likely to depend on the magnitude of the tectonic stress as well as the temperature theoretical considerations suggest a wide range of dependence of seismic wave attentuation and velocity dispersion on the long term tectonic stress this is particularly the case for dislocation mechanisms and warrants careful experimental investigation dislocations and or grain boundaries cause anelastic behavior relaxation peaks when they are pinned or blocked at some points pinning or blocking becomes ineffective at high temperatures and or low frequencies causing a transition to viscoelastic behavior both laboratory and seismological observations of internal friction are dominated by the high temperature background where internal friction increases monotonically with temperature which can be interpreted as a gradual transition to viscoelastic behavior or to a wide distribution of relaxation times however in most experimental studies to date the dislocation densities or the grain sizes were not well controlled making it difficult to identify the attenuation mechanisms and preventing any quantitative applications to earth the need for better characterization of defect microstructures in experimental specimens is emphasized

precambrian continental strain and shear zone patterns south indian case 1 this study addresses the tectonic and mechanical significance of crustal scale strain and shear zone patterns in the south indian precambrian lithosphere it is based on a tectonic map of the dharwar craton derived from landsat imagery existing documentation and our own field observations we document the contrasted responses of the two main parts of the craton to late archean 2 56 2 51 ga shortening the old 3 ga lithosphere of the western dharwar craton stabilized at 2 61 ga underwent moderate shortening and strain localization along spaced shear zones the eastern dharwar craton rejuvenated by late archean juvenile magmatic accretion responded to shortening by flowing laterally against the western dharwar craton shortening operated without significant thickening because of the high buoyancy of the juvenile crust and the very low strength of its mantle lithosphere the late archean crustal scale shear zone network did not accommodate large displacements and only contributed to smoothing out of strain heterogeneities during the latest stages of flow the shear zone pattern only reflects the symmetry of the regional finite strain field of a wide hot orogen it does not result from terrain amalgamation but accompanies crustal flow and does not compare with those of modern active margins which accommodate unidirectional transpression our analysis further suggests that the shear zones bounding the craton did not record large horizontal displacements during final assembly of gondwana but rather transverse shortening and vertical extrusion

new constraints on 1 7 gyr of brittle tectonic evolution in southwestern finland derived from a structural study at the site of a potential nuclear waste repository olkiluoto island abstract the brittle deformation history of southwestern finland was reconstructed through paleostress inversion of fault slip data from outcrops and drill cores at the site of a planned nuclear waste repository seven distinct paleostress states have been resolved spanning 1 7 ga of geological history 1 nw se to nnw sse transpression acted soon after 1 75 ga when conditions leading to brittle deformation were first attained 2 n s to ne sw transpression caused partial reactivation of 1 3 nw se extension followed during the gothian event and at the time of rapakivi magmatism 4 renewed ne sw transtension occurred between 1 60 and 1 30 ga forming the nw se elongated satakunta graben 5 a significant and so far unrecognised phase of ne sw compression has been found to postdate both rapakivi granites and 1 27 ga olivine diabase sills 6 e w transpression followed during the early stages of the mesoproterozoic sveconorwegian orogeny and predated 7 almost coaxial e w extension of the late sveconorwegian orogenic collapse our results prove that the shield s crust fragmentation i e generation of new fractures was short lived in comparison to its jostling phase i e reactivation of old fractures the shield thus reached structural maturity with respect to its saturation with brittle structures already in the late mesoproterozoic we predict that that future stress changes will most probably be accommodated by reactivation of the existing and inherited brittle structures rather than by formation of new deformation zones

seismic attenuation and mantle wedge temperatures in the alaska subduction zone 1 anelastic loss of seismic wave energy or seismic attenuation 1 q provides a proxy for temperature under certain conditions the q structure of the upper mantle beneath central alaska is imaged here at high resolution an active subduction zone where arc volcanism is absent to investigate mantle thermal structure the recent broadband experiment across the alaska range beaar provides the first dense broadband seismic coverage of this region the spectra of p and sh waves for regional earthquakes are inverted for path averaged attenuation operators between 0 5 and 20 hz along with earthquake source parameters these measurements fit waveforms significantly better when the frequency dependence of q is taken into account and in the mantle frequency dependence lies close to laboratory values inverting these measurements for spatial variations in q reveals a highly attenuating wedge with q 500 assuming frequency dependence comparison with p results shows that attenuation in bulk modulus is negligible within the low q wedge as expected for thermally activated attenuation mechanisms bulk attenuation is significant in the overlying crust and subducting plate indicating that q must be controlled by other processes the shallowest part of the wedge shows little attenuation as expected for a cold viscous nose that is not involved in wedge corner flow overall the spatial pattern of q beneath alaska is qualitatively similar to other subduction zones although the highest wedge attenuation is about a factor of 2 lower the q values imply that temperatures exceed 1200 c in the wedge on the basis of recent laboratory based calibrations for dry peridotite these temperatures are 100 150 c colder than we infer beneath japan or the andes possibly explaining the absence of arc volcanism in central alaska

age and tectonic framework of structurally controlled palaeoproterozoic gold mineralization in the hame belt of southern finland abstract a number of gold occurrences in the palaeoproterozoic svecofennian domain of southern finland show a clear structural control on mineralization and are classified as orogenic gold samples of pre syn and late to post mineralization rocks taken from two gold prospects in the somero tammela gold zone of the hame belt have been studied to gain absolute ages of mineralization by using sims u pb of zircon and titanite and ar ar single grain step heating of hornblende and biotite gold mineralization in the somero tammela gold zone is closely related to quartz veins and controlled by wsw ene to sw ne trending shear zones and faults formed during prolonged dextral oblique contraction in the late stages of svecofennian orogenic evolution zircons from a felsic volcanic host rock give an age of 1881 3 ma interpreted as the crystallization age of the rock and providing a maximum age for mineralization inherited zircons in a gold bearing quartz vein with the youngest zircon rim of 1832 15 ma give a maximum age of quartz vein emplacement a 1786 7 ma mean 207pb 206pb age of a pegmatitic quartz feldspar dyke providing a minimum age for mineralization is interpreted as reflecting lead loss whereas an 1815 4 ma zircon age is probably closer to the crystallization age of the dyke this age overlaps with 1805 4 5 4 ma 39ar 40ar age of hornblende from a quartz vein 1 78 and 1 76 ga 39ar 40ar biotite cooling ages mark the minimum age for mineralization in summary the age of mineralization can be estimated at ca 1 82 1 79 ga which is in good agreement with the age of dextral shearing in southern finland the controlling structures wsw ene to sw ne dextral shear zones and nw se oriented faults formed during approximately wnw ese oriented sub horizontal shortening during crossing ductile brittle boundary conditions the steep shear zones and faults and the intersection lines of the two sets impart a directional permeability allowing channelling of the mineralizing fluids in these structures this was supported by prolonged dextral oblique contraction enabling continuous or episodic slip along the faults and shear zones the late svecofennian tectonic episode could be described as an extension contraction cycle in a convergent plate margin setting above a retreating subduction zone oblique extension 1 86 1 83 ga caused by slab rollback was followed by a rapid switch to oblique contraction at ca 1 82 ga induced by arrival of positively buoyant crustal fragments e g oceanic ridge oceanic plateau or continental sliver at the subduction zone the contractional period characterized by dextral transpression provided a favourable setting for orogenic gold mineralization the termination of the svecofennian orogenic evolution in finland after 1 79 ga might be caused by lock up of the subduction zone and slab break off the striking nw se alignment of orogenic type gold occurrences in the svecofennian domain of southern finland suggests that the late svecofennian 1 82 1 79 ga gold mineralizing event may be regionally more widespread and of greater importance than hitherto expected

seismic attenuation tomography of the mariana subduction system implications for thermal structure volatile distribution and slow spreading dynamics 1 the anelastic structure of a subduction zone can place first order constraints on variations in temperature and volatile content we investigate seismic attenuation across the western pacific mariana subduction system using data from the 2003 2004 mariana subduction factory imaging experiment this 11 month experiment consisted of 20 broadband stations deployed on the arc islands and 58 semibroadband ocean bottom seismographs deployed across the fore arc island arc and back arc spreading center we compute amplitude spectra for p and s arrivals from local earthquakes and invert for the path averaged attenuation for each waveform along with the seismic moment and corner frequency for each earthquake additionally we investigate earthquake source parameter assumptions and frequency dependent exponents ranging from 0 to 0 6 tomographic inversion of nearly 3000 t estimates at 0 27 for 2 d qp 1 and qp qs structure shows a 75 km wide columnar shaped high attenuation anomaly with qp 43 60 beneath the spreading center that extends from the uppermost mantle to 100 km depth a weaker high attenuation region qp 56 70 occurs at depths of 50 100 km beneath the volcanic arc and the high attenuation regions are connected at depths of 75 125 km the subducting pacific plate is characterized by low attenuation at depths greater than 100 km but high attenuation is found in the plate between 50 and 100 km depth the fore arc shows high attenuation near the volcanic arc and beneath the serpentinite seamounts in the outer fore arc qs structure is less well resolved than qp because of a smaller data set but qp qs ratios are significantly less than 2 throughout the study region as temperatures estimated from qs 1 are unusually high we interpret the arc and wedge core anomalies as regions of high temperature with enhanced q 1 due to hydration and or melt the slab and fore arc anomalies as indicative of slab derived fluids and or large scale serpentinization and the columnar shaped high qp 1 anomaly directly beneath the back arc spreading center as indicative of a narrow region of dynamic upwelling and melt production beneath the slow spreading ridge axis

unusual mantle poisson s ratio subduction and crustal structure in central alaska 1 subduction zones alter the upper plate in a variety of ways including metasomatism via slab derived fluids and accretion of exotic terranes these processes should produce distinctive seismic signatures as seismic velocities are sensitive to compositional variations a new method is developed to estimates depths to interfaces and poisson s ratio vp vs ratio for dipping layered structures by stacking teleseismic receiver functions it also estimates crustal vp vs and thickness using this method we analyze a broadband data set from the central alaska subduction zone to seek evidence for mantle wedge composition crustal thickness varies from 27 to 45 km and crustal vp vs indicates an intermediate to mafic composition expected for these accreted island arc terranes inversions for mantle structure confirm the presence of a 15 20 km thick low velocity zone atop of the downgoing plate to 130 km depth perhaps subducted crust of an exotic terrane vp vs in the mantle wedge allows for 15 15 serpentinization where the slab is 80 km deep and the wedge is cold where the slab is deeper vp vs for the mantle wedge is unusually low 1 7 this value is lower than predicted for any common mantle mineral and may indicate significant quantities of quartz provided that past physical properties measurements are not grossly in error these results suggest that the mantle wedge in subduction zones can occasionally differ substantially in bulk composition from normal subcrustal mantle perhaps because collisions of exotic terranes cause tectonic mixing of crustal material into the mantle

sources of svecofennian granitoids in the light of ion probe upb measurements on their zircons abstract the presence of 1 91 1 93 ga old granitoids at the archean proterozoic boundary along the raahe ladoga zone in finland has been demonstrated on various occasions these rocks have been considered to represent juvenile crustal material as their e nd values are markedly positive however as svecofennian metasediments contain detrital zircons derived from a ca 2 ga old source the possibility has existed that the 1 92 ga age may have been a mixture between 2 and 1 89 ga old zircon populations as such mixing would not markedly affect their neodymium isotopic properties also some syntectonic 1 89 ga old svecofennian granitoids contain heterogeneous zircon populations but it has been impossible to determine the age and origin of the older zircons by conventional methods nordsim ion probe results on three samples from the 1 92 ga age group confirm the earlier conclusions especially important is that no zircons older than 1 95 ga were detected in the 1 92 ga group samples thus the 1 92 ga event was the beginning of the formation of new continental crust in the primitive svecofennian island arc and these granitoids formed by partial melting of basaltic magmas derived from a depleted mantle source one sample also contains a younger zircon population formed during the orogenic culmination at 1 89 ga in contrast one grain from a sample representing the 1 89 ga age group contains an archean core which is considered to represent sedimentary detritus assimilated during either magma formation or intrusion while the results prove the true igneous nature of the 1 92 ga event they also rule out these rocks as a possible provenance for the ca 2 ga old zircons encountered in the svecofennian metaturbidites thus there is still no direct evidence from granitoid rocks for an extensive svecofennian protocrust the existence of which has been postulated on the basis of geochemical and sm nd isotopic data

relationship between anisotropy of p and s wave velocities and anisotropy of attenuation in serpentinite and amphibolite velocities and q values of p and s waves as functions of pressure and temperature at 100 and 600 mpa are presented for a serpentinite and an amphibolite both rocks exhibit a strong lattice preferred orientation lpo of the major mineral phases antigorite and hornblende respectively velocities and q values increase with pressure the rate of increase is different in the three orthogonal directions normal and parallel to foliation and lineation and closely related to progressive closure of microcracks increasing temperature decreases velocities and q values only slightly as long as thermal cracking is prevented by the applied confining pressure substantial anisotropy of velocities and q in p and s waves is observed in both rocks but is found to be different in origin anisotropy of p and s wave velocities is highest at low pressure and basically caused by constructive interference of effects related to oriented microcracks and to the lpo of major minerals increasing confining pressure decreases velocity anisotropy at a smaller and smaller rate the residual anisotropy of p and s wave velocities shear wave splitting at high confining pressure is mainly a result of preferred mineral orientation by contrast anisotropy of q is very low at low confining pressure and markedly enhanced as pressure is increased at high confining pressure substantial anisotropy of q in p waves is apparent but reversed from that of p wave velocities qp is highest in the direction normal to the foliation plane whereas vp and vs is lowest in this direction the generation of a pronounced anisotropy of qp by increasing pressure is due to a directionally dependent increase of contact areas on the oriented grain boundaries of the platy minerals defining the foliation the increase of q with pressure in the direction normal to foliation is mainly caused by the decrease of energy loss due to compressive strain relative to shear strain the reverse is true for the x and y directions serpentinite and x direction amphibolite parallel to the foliation plane

contrasting styles of phanerozoic and precambrian continental collision abstract there are differences in the style of collisional orogens between the phanerozoic and the precambrian most notably the appearance of blueschists and ultrahigh pressure metamorphic uhpm rocks in the geological record since the late neoproterozoic whereas these rocks are absent from older orogens understanding collisional orogenesis in the context of present day values for ambient upper mantle temperature and radiogenic heat production provides a reference from which to extrapolate back to conditions in the precambrian to evaluate differences in the way phanerozoic and precambrian collisional orogens develop a series of experiments was run using a 2 d petrological thermomechanical numerical model in which the collision of spontaneously moving continental plates was simulated for values of ambient upper mantle temperature and radiogenic heat production increasing from those appropriate to the present day thus models of modern collisional orogens involving different modes of exhumation of uhpm rocks were extrapolated back to conditions appropriate for the precambrian based on these experiments an increase of the ambient upper mantle temperature to 80 100 k above the present day value leads to two distinct modes of collision that are different from the modern collision regime and for which the terms truncated hot collision regime strong mafic lower continental crust and two sided hot collision regime weak felsic lower continental crust are proposed some proterozoic orogens record post extension thickening to generate counter clockwise metamorphic p t paths followed by slow close to isobaric retrograde cooling such as occurred in the paleoproterozoic khondalite belt in the north china craton and the late mesoproterozoic early neoproterozoic eastern ghats province part of the eastern ghats belt of peninsular india these orogens have similarities with the truncated hot collision regime in the numerical models assuming subsequent shortening and thickening of the resulting hot lithosphere other proterozoic orogens are characterized by clockwise looping metamorphic p t paths and extensive granite magmatism derived from diverse crustal and subcontinental lithospheric mantle sources these orogens have similarities with the two sided hot collision regime in the numerical models both regimes are associated with shallow slab breakoff that precludes the formation of uhpm rocks the temperature of the ambient upper mantle where this transition in geodynamic regimes occurs corresponds broadly to the neoproterozoic era

paleoproterozoic lateritic paleosol ultra mature mature quartzite meta arkose successions in southern fennoscandia intra orogenic stage during the svecofennian orogeny abstract a paleoproterozoic lateritic paleosol overlain by ultra mature mature quartzite is reported for the first time from three localities in the svecofennian province in southern finland these are important indicators for warm paleoclimate and continental paleoenvironment ultra mature quartzites above the lateritic paleosols are overlain by matrix supported conglomerates followed by mature quartzites these latter rocks are related to incipient rifting followed by main rift stage starting with increasingly more immature meta arkose we present a tectonic model in which exhumation after 1 88 1 87 ga contraction unroofed plutonic rocks volcanism and exhumation related erosion produced deposition of both volcanic and sedimentary material weathering of these rocks and deposition of ultra mature mature quartzites took place between 1 87 and 1 85 1 84 ga followed by rifting at 1 84 ga tectonic slicing during basin inversion thrust the pre rift quartzites and rift related volcanic and sedimentary rocks over older mid crustal rocks and produced a low angle stacking system to accommodate voluminous magmatism at deeper crustal levels and widespread sedimentation at the surface we propose that exhumation due to faulting followed by stabilization of the upper crust 0 15 20 km occurred simultaneously as the detached hot middle and lower crust experienced vertical shortening via ductile flow and extrusion the ca 1 85 ga paleosol quartzite successions in southern fennoscandia have many similarities with the widespread ca 1 7 ga paleosol quartzite successions in the united states both are intra orogenic formed on hot orogens and have simultaneous magmatism at deeper crustal levels and sedimentation at the surface accretion type models presented for southern fennoscandia at 1 87 1 80 ga are not compatible with the available data and either far field setting during outboard subduction or a newly formed continental interior setting are proposed anyhow tectonic models proposed for southern fennoscandia between 1 87 and 1 80 ga should take into account the occurrence of lateritic paleosols and ultra mature quartzites

reactive monazite and robust zircon growth in diatexites and leucogranites from a hot slowly cooled orogen implications for the palaeoproterozoic tectonic evolution of the central fennoscandian shield sweden monazite in melt producing poly metamorphic terranes can grow dissolve or reprecipitate at different stages during orogenic evolution particularly in hot slowly cooling orogens such as the svecofennian owing to the high heat flow in such orogens small variations in pressure temperature or deformation intensity may promote a mineral reaction monazite in diatexites and leucogranites from two svecofennian domains yields older coeval and younger u pb sims and emp ages than zircon from the same rock as zircon precipitated during the melt bearing stage its u pb ages reflect the timing of peak metamorphism which is associated with partial melting and leucogranite formation in one of the domains the granite and diatexite belt zircon ages range between 1 87 and 1 86 ga whereas monazite yields two distinct double peaks at 1 87 1 86 and 1 82 1 80 ga the younger double peak is related to monazite growth or reprecipitation during subsolidus conditions associated with deformation along late orogenic shear zones magmatic monazite in leucogranite records systematic variations in composition and age during growth that can be directly linked to th u ratios and preferential growth sites of zircon reflecting the transition from melt to melt crystallisation of the magma in the adjacent ljusdal domain peak metamorphism in amphibolite facies occurred at 1 83 1 82 ga as given by both zircon and monazite chronology pre partial melting 1 85 ga contact metamorphic monazite is preserved in spite of the high grade overprint by combining structural analysis petrography and monazite and zircon geochronology a metamorphic terrane boundary has been identified it is concluded that the boundary formed by crustal shortening accommodated by major thrusting

radioactivity heat flow and rifting of the indian continental crust the relationship between heat flow q and heat production a in india is quite different for the northern and southern parts of the peninsula in the north locations in a variety of tectonic settings including gondwana rifts show a linear relationship between q and a with a common reduced heat flow q about 40 mwm 2 in southern india the heat production of surface rocks is greater than is expected for the measured heat flow relative to other continental shields g is high in northern india and about average in the south apparently the indian crust evolved with a content of radioactive elements higher than in most shields and the southern part of the peninsula has undergone regional depletion in the lower crust and possibly upper mantle this depletion may coincide with a regional granulite event about 2 500 m y ago the high heat flow and heat production in northern india may explain several unique characteristics of the indian peninsula one feature is the continued reactivation of rif

svecofennian magmatic and metamorphic evolution in southwestern finland as revealed by u pb zircon sims geochronology abstract zircons from six samples collected from igneous and metamorphic rocks were dated using the nordsim ion microprobe in order to investigate the tectonic evolution of the palaeoproterozoic svecofennian orogen in southwestern finland these rocks represent pre collisional collisional and post collisional stages of the orogeny the ion microprobe results reveal two age groups of granodioritic tonalitic rocks the intrusions have different tectonic settings the orijarvi granodiorite represents pre collisional 1 91 1 88 ga island arc related magmatism and yielded an age of 1898 9 ma whereas the collision related masku tonalite was dated at 1854 18 ma the latter age accords with more accurate previous conventional zircon age data and constrains the emplacement age of collisional granitoids to 1 87 ga this is interpreted to reflect the collision between the southern svecofennian arc complex with the central svecofennian arc complex and the formation of a suture zone between them during d2 deformation granulite facies metamorphism in the turku area was dated at 1824 5 ma using zircons from leucosome in the lemu metapelite this age constrains d3 folding related to post collisional crustal shortening in this area crustal melting continued until 1 81 ga as indicated by the youngest leucosome zircons and metamorphic rims of enderbite zircons new metamorphic zircon growth took place in older granitoids at granulite facies but not at amphibolite facies detrital zircons with ages between 2 91 and 1 97 ga were found in the mesosome of the lemu metapelite and 2 64 1 93 ga inherited cores were found in the 1 87 ga masku tonalite

complex patterns of fluid and melt transport in the central andean subduction zone revealed by attenuation tomography abstract we present a high resolution 3 d model of p wave attenuation q p 1 for the central andean subduction zone data from 1500 mostly intermediate depth 60 250 km earthquakes recorded at three temporary seismic networks covering the forearc arc and backarc around 23 s were used for tomographic inversion the forearc is characterised by uniformly high q p values indicating low temperature rocks in accordance with low surface heat flow values prominent low q p anomalies are found beneath the magmatic arc and the backarc in the crust and mantle continuous regions of low q p connect earthquake clusters at 100 km and 200 km depth with zones of active volcanism in the arc and backarc fluids fluxed from the subducted oceanic lithosphere into the overlying mantle wedge where they induce melting explain our observations we propose that low q p regions indicate source and ascent pathways of metamorphic fluids and partial melts ascent of fluids and melts as imaged by seismic q p are not vertical as is often implicitely assumed instead sources of fluids are located at different depth levels and ascent paths are complex and exhibit significant variation within the study area the largest quaternary backarc volcano cerro tuzgle is fed by mantle melts which are imaged as a plume of low q p material that reaches to the strong earthquake cluster at 200 km depth

late glacial and post glacial deposition in a large low relief epicontinental basin the northern baltic sea this paper presents a model of late glacial and post glacial deposition for the late neogene sedimentary succession of the archipelago sea in the northern baltic sea four genetically related facies associations are described i an ice proximal acoustically stratified draped unit of glaciolacustrine rhythmites ii an onlapping basin fill unit of rotated rhythmite clasts in an acoustically transparent to chaotic matrix interpreted as debris flow deposits iii an ice distal acoustically stratified to transparent draped unit of post glacial lacustrine weakly laminated to homogeneous deposits and iv an acoustically stratified to transparent unit of brackish water organic rich sediment drifts the debris flow deposits of the unit 2 pass laterally into slide scars that truncate the unit 1 they are interpreted to result from a time interval of intense seismic activity due to bedrock stress release shortly after deglaciation of the area ice berg scouring and gravitational failure of oversteepened depositional slopes may also have contributed to the debris flow deposition comparisons to other late neogene glaciated basins such as the hudson bay or glacial lakes formed along the laurentide ice sheet suggest that the archipelago sea succession may record development typical for the deglaciation phase of large low relief epicontinental basins the carboniferous permian glacigenic dwyka formation in south africa may provide an ancient analogue for the studied succession chronological control for the studied sediments is provided by the independent palaeomagnetic and ams 14c dating methods in order to facilitate dating of the organic poor early post glacial deposits of the northern baltic sea the 10 000 year long lake nautajarvi palaeomagnetic reference chronology ojala saarinen 2002 is extended by 1200 years

strong along arc variations in attenuation in the mantle wedge beneath costa rica and nicaragua attenuation structure in the central american subduction zone was imaged using local events recorded by the tomography under costa rica and nicaragua array a 20 month long deployment july 2004 until march 2006 of 48 seismometers that spanned the fore arc arc and back arc regions of nicaragua and costa rica p and s waveforms were inverted separately for the corner frequency and moment of each event and for the path averaged attenuation operator t of each event station pair assuming attenuation is slightly frequency dependent 0 27 then tomographic inversions were performed for s and p attenuation q s 1 and q p 1 since p wave amplitudes reflect both shear and the bulk moduli tomographic inversions were also performed to determine shear and bulk attenuation q s 1 and q 1 the loss of energy per cycle owing to shearing and uniform compression respectively damping and other inversion tomographic parameters were systematically varied as is typical in subduction zone attenuation studies a less attenuating slab upper plate and wedge corner and a more attenuating mantle wedge were imaged in addition first order differences between the mantles beneath nicaragua and costa rica were observed the slab in nicaragua is more attenuating than the slab in costa rica a larger zone of higher shear attenuation also characterizes the nicaraguan mantle wedge within the wedge maximum attenuation values at 1 hz correspond to qs 38 73 beneath nicaragua and qs 62 84 beneath costa rica and average values are qs 76 78 and qs 84 88 respectively attenuation variations correlate with along arc trends in geochemical indicators that suggest that melting beneath nicaragua occurs at more hydrated conditions and possibly to greater extents and depths relative to northern costa rica shear attenuation dominates over bulk attenuation in the well resolved regions of the wedge the more extensive zones of greater shear attenuation observed in the nicaraguan wedge could be explained by higher temperatures and or greater hydration but comparison with petrological data suggests that hydration variations play a larger role average wedge attenuation values are comparable to estimates for the andes and japan greater than those for alaska and less than those for tonga lau