geochemistry u pb geochronology and sm nd data from the paleoproterozoic western finland supersuite a key component in the coupled bothnian oroclines abstract the western finland wf supersuite characterized by supracrustal rocks and intruding plutonic rocks forms the core component of the coupled bothnian oroclines in the central part of the svecofennian orogen we present here geochemical isotope data detrital zircon age data on sedimentary rocks and age data on granitoid plutons within the wf supersuite in the pirkanmaa pb and pohjanmaa pob belt representing the southernmost limb of the southern and the hinge zone of the northern bothnian orocline respectively the tampere belt tb in the north and hame belt hb in the south bound the pb basaltic to picritic volcanic rocks in the wf supersuite with a proposed eruption age at ca 1 91 ga 1 92 1 90 ga show locally pillow structures and are typically associated with black shales cherts carbonate bearing rocks based on geochemistry and sm nd data a variable morb wpb type origin is evident and an evolution scenario from earlier wpb type rifting to more morb affinity ocean sea opening is proposed detrital zircon age distributions from sedimentary rocks with 1 91 ga maximum depositional age in the tb pb and hb have been compared and typically the age distributions of

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

horizontal structures in granulite terrains a record of mountain building or mountain collapse in many high temperature 800 c granulite terrains the development of characteristic horizontal structures occurred during the metamorphic culmination and was followed by isobaric cooling the absolute magnitude of isobaric cooling commonly 300 c implies cooling intervals of the order of the thermal time constant of the continental lithosphere 100 m y such prolonged isobaric cooling implies that no significant erosional denudation followed the development of the horizontal structures and thus precludes the prograde deformation being responsible for significant crustal thickening rather the prograde deformation more probably records bulk crustal thinning during extensional collapse of a previously thickened crust possibly triggered by detachment of a thickened thermal boundary layer at the base of the lithosphere

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

zircon and monazite geochronology of deformation in the pielavesi shear zone finland multistage evolution of the archaean proterozoic boundary in the fennoscandian shield the raahe ladoga shear complex is a major crustal structure representing the archaean palaeoproterozoic boundary in the fennoscandian shield the complex developed during the svecofennian orogeny c 1 9 1 8 ga beginning with regional thrust tectonic phases d 1 and d 2 followed by large scale shearing events d 3 and d 4 the pielavesi shear zone is a vertical north south trending shear zone within the raahe ladoga shear complex formed during regional d 3 shearing and later reactivated during the regional d 4 phase three north south trending elongate granitoid intrusions were selected as representative of silicic melts that intruded the transtensional pielavesi shear zone during the regional d 3 phase the oriented magmatic fabric of the granitoids indicates that they intruded coeval to the deformation event the zircon u th pb secondary ionization mass spectrometry sims ages of these intrusions 1888 4 1884 6 and 1883 5 ma overlap within error and provide a direct age for the regional d 3 deformation e hf t 1 1 to 3 4 and e nd t 1 2 to 0 4 values from these granitoids are both consistent with a predominantly juvenile source affected by a minor archaean component u th pb sims analyses of metamorphic monazite formed within a crosscutting blastomylonite provide an age for the regional d 4 phase and associated fluid activity of 1793 3 ma supplementary material analytical methods and tabulated analytical data are available at https doi org 10 6084 m9 figshare c 3498501

the mechanism of ascent and emplacement of granite magma during transpression a syntectonic granite paradigm we propose a model for syntectonic ascent and emplacement of granite magma based on structural relations in part of the northern appalachians in the study area in western maine strain was distributed heterogeneously during devonian acadian transpression metasedimentary rocks migmatites at high grades record two contrasting types of finite strain in zones that alternate across strike rocks in both types of zones have a penetrative moderately to steeply ne plunging mineral elongation lineation defined by bladed muscovite fibrolite sillimanite at high grades in straight belts of enhanced deformation rocks have s l fabrics that record apparent flattening to plane strain apparent flattening zones afzs but rocks between these belts have l s fabrics that record apparent constriction apparent constriction zones aczs at metamorphic grades above the contemporary solidus rocks in afzs developed stromatic structure in migmatite which suggests that percolative flow of melt occurred along the evolving flattening fabric stromatic migmatites are intruded by concordant to weakly discordant m scale composite sheet like bodies of granite to suggest magma transport in planar conduits through the afz rocks inhomogeneous migmatite is found in the intervening aczs which suggests migration of partially molten material through these zones en masse probably by melt assisted granular flow inhomogeneous migmatites are intruded by irregular m scale bodies of granite that vary from elongate to sub circular in plan view and seem cylindrical in three dimensions these bodies apparently plunge to the northeast parallel to the regional mineral elongation lineation to suggest magma transport in pipe like conduits through the acz rocks we postulate that the form of magma ascent conduits was deformation controlled and was governed by the contemporaneous strain partitioning magma ascent in planar and pipe like conduits through migmatites is possible because oblique translation during contraction displaces isotherms upward in the orogenic crust to form a thermal antiform within this hot corridor it is the difference in temperature between melt producing reactions in the anatectic zone and the wet solidus for granite melt that enables magma to migrate pervasively up through the orogenic crust without congealing heat advected with the migrating melt promotes amplification of the thermal antiform in a feedback relation that extends the zone of plastic deformation and pervasive melt migration to shallower levels in the crust at the wet solidus we suggest melt flows obliquely toward axial culminations in the thermal antiform which are sites of melt accumulation and perturbations from which magma may escape to form plutons batches of melt that escape from these perturbations may be trapped by tectonic structures higher in the crust or ascent may become inhibited with decreasing depth by thermal arrest and solidification if the rate of arrival of subsequent melt batches exceeds the rate of crystallization at the site of pluton construction melt pressure ultimately may lead to sub horizontal magma fracture or viscous flow of wall rocks may allow lateral spreading the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduit as the thermal antiform decays the height of sub solidus crust that separates the deepest part of the pluton from the anatectic zone increases consequently pluton inflation declines and solidification leads to infilling of the magma feeder channel to form a root zone that passes downward into migmatite which may explain the difficulty of determining precisely the floor to these deeper segments of large plutons using gravimetry

emplacement mechanisms of late orogenic granites structural and geochemical evidence from southern finland the country rock in southern finland formed mainly during the svecofennian orogeny ca 1 9 ga ago the middle and lower crust was partially melted 1 83 ga ago due to crustal thickening and subsequent extension during this event s type migmatites and granites were formed along a 100 500 km zone this late svecofennian granite migmatite zone lsgm zone is a large crustal segment characterised by roughly e w trending sub horizontal migmatites and granites combined ductile e w shear movements and nnw sse compressional movements defined a transpressional tectonic regime during the emplacement partial melts that moved through the crust pooled as granite sheets or froze as migmatites major transpressive shear zones border the lsgm zone which forms a tectonic and metamorphic zone that crosscuts the earlier svecofennian granitoids based on field observations and geochemical data from two sets of outcrops we show that the great volumes of late orogenic granites and migmatites in southern finland were transported and emplaced as small chemically variable batches possibly extracted from different protoliths these melt batches were transported along repeatedly activated channels and collected at some horizontal level in the crust in the nagu area the melt batches were trapped under a roof layer of amphibolite and the whole complex was synchronously folded into open folds with steep axial surfaces and e w trending fold axes the sheets of microcline granite are in places strongly sheared the microcline phenocrysts are imbricated and subsequent deformation of the microcline phenocrysts indicates syn tectonic movements of the layers as well as a syn tectonic mechanism for the late magmatic fractionation depending on the degree of crystallisation of the individual melt batches during shearing at different intensities the granites have slightly different appearances some sheared zones show a cumulate like trace element geochemistry indicating that melt fractions were expelled from the system producing layers of deformation enhanced fractionated granites and cumulate layers our interpretation is that the nagu area shows shear assisted fractionation mechanisms in granitic melts and that similar processes are responsible for the fractionation trends seen in the sub horizontal sheeted granites in hameenlinna at higher levels in the crust

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

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

the structure and timing of lateral escape during the scandian orogeny a combined strain and geochronological investigation in finnmark arctic norwegian caledonides abstract determining the timing duration and mechanism of tectonic events within an orogenic cycle such as ocean subduction continent continent collision or gravitational collapse is challenging especially in ancient orogenic belts variations in the tectonic transport direction however can be used as a guide to these stages of orogeny while thrust sheets within the caledonian allochthon in north norway were emplaced broadly eastwards perpendicular to the trend of the orogen many features indicate material transport in other orientations one dominant feature of the mageroy nappe sitting above and infolded with the kalak nappe complex is the development of a strong n s lineation orthogonal to the main transport direction strain measurements in part determined by a new method are used in the context of the regional structural data to identify the critical stage in orogeny when compressional forces are balanced by orogen parallel lateral escape quantitative 3 d strain estimation in the mageroy nappe indicates prolate deformation with c 50 horizontal shortening parallel to the thrusting direction e w and c 200 extension along the orogenic strike n s with c 30 vertical shortening temporal constraint on this fabric is provided by ar ar isotopic analysis of undeformed white mica in cross cutting granitic pegmatites these data show that prolate deformation occurred before the white mica cooling age of 416 4 ma while the previously determined depositional age of the hellefjord schist indicates that it occurred after 438 4 ma a granitic pegmatite that intruded the hellefjord schist after an initial deformation phase but during or prior to a later deformation has been dated at 431 2 ma by u pb zircon a previous lower age constraint on this deformation of 428 5 ma is given by metamorphic zircon overgrowths on fractured grains these results constrain the continental collision between baltica and laurentia in finnmark to the interval c 431 428 ma placed in a regional context these results indicate that lateral escape was orthogonal to the thrusting direction and occurred during the continent continent collision stage in the scandian orogeny before gravitationally driven collapse

origin of a large scale fold nappe in the montagne noire variscan belt france abstract detailed structural analysis of a large recumbent fold in low grade palaeozoic sediments of the montagne noire and the previously established tectonometamorphic history of its crystalline substratum indicate a two stage development history first relatively homogeneous crustal thickening produced folds with steep axial planes associated with a regional cleavage s1 subsequently these structures became rotated and amplified in a subhorizontal flow with pure shear and simple shear components related to low angle thrusting the development of an associated horizontal crenulation cleavage s2 with a component of vertical shortening of s1 plus evidence for a synchronous decompression of autochthonous units allows this flow regime to be placed in the context of a thrust wedge undergoing gravitational spreading above a continuously contracting footwall decoupling of an extending thrust wedge from a contracting footwall by a basal thrust with low shear resistance explains bulk shortening uplift and exhumation of mid to lower crustal autochthonous basement during thrusting and recumbent folding at higher levels structural relationships in the southern montagne noire demonstrate that traditional tectonic units recognized in this area the so called nappes are in fact unrelated to the recumbent folding as inferred previously these nappes postdate not only the recumbent folding but also a later phase of refolding d3 associated with the formation of a gneiss dome the traditional nappe division can be simplified to a hanging wall and footwall domain of a major decollement that formed during a second syn collisional collapse d4 associated with the formation of intermontane stephanian basins the late timing of the nappes and simplified tectonic structure of the southern montagne noire imply a thinner d2 recumbent fold structure with only one major anticline syncline pair and is in better agreement with the low metamorphic grade of the area

penetrative ductile deformation and amphibolite facies metamorphism prior to 1851 ma in the western part of the svecofennian orogen fennoscandian shield the svecofennian orogen in the fennosandian shield consists of several palaeoproterozoic meta igneous provinces that generally young progressively southwards and westwards the understanding of the tectonic evolution of these different provinces is linked to the ability to constrain the timing of ductile deformation and metamorphism within them in the western part of the orogen in central sweden ductile deformation and metamorphism is conventionally thought to have occurred after 1 85 ga and mainly around 1 82 1 80 ga at forsmark in central sweden we have dated two undeformed granite dykes that cross cut the tectonic fabric in their host rocks to 1851 15 and 1855 6 ma respectively since the former shows an unequivocal field relationship with respect to the tectonic fabric we establish that penetrative ductile deformation under amphibolite facies metamorphic conditions occurred prior to 1851 5 ma this u pb zircon minimum age for the timing of ductile deformation is supported by a u pb titanite age of 1844 4 ma these data as well as the field relationships at forsmark indicate that an early phase of penetrative deformation and metamorphism affected the svecofennian bedrock in the western part of the fennoscandian shield and was overprinted after 1851 5 ma by spatially more confined deformational events in conjunction with earlier results our data help to establish the presence of tectonic domains with contrasting tectonothermal histories in the western part of the svecofennian orogen c 2006 elsevier b v all rights reserved less

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

the accretionary svecofennian orogen insight from the babel profiles abstract the babel profiles b c 1 7 form a 1200 km long nearly continuous cross section through the svecofennides the near vertical marine reflection profiles display a wide range of crustal structures that can be associated with both the accretionary svecofennian orogeny 1 96 1 75 ga and the following subjotnian and jotnian rift stages 1 65 1 11 ga the svecofennian accretionary orogeny took place when a number of micro plates with island arc affinities and surface expression of a large archipelago accreted to the continental karelian plate some of the accreting terranes seem to have had older cores that have acted as crustal indentors during the collision babel profiles 3 and 4 image a series of collisional terrane boundaries between karelian continental margin savo arc sa and central finland arc cfa in the north the karelian margin has been both over and underthrust by the savo arc cfa comprises a folded thrust package on top of a continental nucleus and the savo arc the associated subduction zone and accretionary prism are interpreted to lie to the south underneath the bothnian basin area where prominent ne dipping lower to middle crustal reflections are found along babel profiles 1 and 4 an oblique collision of the central finland arc and the continent resulted in the development of the strike slip fault on the young hot savo arc babel profiles 1 6 7 and c image the internal architecture of the southern and central finland arcs the unusually thick crust 55 60 km hosts unreflective high density mafic intrusions acting as a crustal indentor a highly reflective antiform structure developed on the southern side of it southern finland arc complex sfa is an imbrication structure comprising stacked slices of arc related crust on an older continental nucleus bergslagen nucleus prior to the collision the sfa suffered from gravitational collapse during which the crust was thinned profile b images the architecture of the central and southern swedish svecofennides the sormland terrane sob is interpreted as the accretionary prism of the southern finland arc squeezed between the svecofennian collage in the north and the advancing continent to the south the southern continent island arc is characterised by ne dipping crustal reflections and moho offsets as well as step wise increasing thickness of the crust after the final collision large volumes of mantle derived material intruded the crust as large mushroom shaped plutons they are interpreted as the heat source for the tib magmatism in southern and western scandinavia

a new interpretation of decomposition products of serpentine under shock compression dense hydrous magnesium silicates dhmss may play an important role in water transport during planetary accretion and as water reservoirs in the earth s deep mantle we show that the dynamic decomposition products of antigorite mg3si2o5 oh 4 can be interpreted as containing the newly discovered dense hydrous silicate phase h mgsio4h2 the hugoniot for phase h was calculated based on the hugoniots for its constituent oxides and equation of state data derived from first principles calculations the measured antigorite hugoniot previously suggested to decompose into high pressure phases without generating fluid h 2o was compared with those derived from calculations involving phase h sound velocity data were also compared to confirm that the dynamic breakdown product of antigorite at pressures above 40 gpa is most likely phase h plus mgo without formation of fluid h 2o

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

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

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

evolution of north himalayan gneiss domes structural and metamorphic studies in mabja dome southern tibet field structural and metamorphic petrology investigations of mabja gneiss dome southern tibet suggest that contractional extensional and diapiric processes contributed to the structural evolution and formation of the domal geometry the dome is cored by migmatites overlain by sillimanite zone metasedimentary rocks and orthogneiss metamorphic grade diminishes upsection and is defined by a series of concentric isograds evidence for three major deformational events two older penetrative contractional and extensional events and a younger doming event is preserved metamorphism migmitization and emplacement of a leucocratic dike swarm were syntectonic with the extensional event at mid crustal levels metamorphic temperatures and pressures range from 500 8c and 150 450 mpa in chloritoid zone rocks to 705 65 8c and 820 100 mpa in sillimanite zone rocks we suggest that adiabatic decompression during extensional collapse contributed to development of migmatites diapiric rise of low density migmatites was the driving force at least in part for the development of the domal geometry the structural and metamorphic histories documented in mabja dome are similar to kangmar dome suggesting widespread occurrence of these events throughout southern tibet q 2004 elsevier ltd all rights reserved

tectonic controls on metamorphism partial melting and intrusion timing and duration of regional metamorphism and magmatism in the nigde massif turkey protracted heating and melting of orogenic middle crust followed by emplacement of a syn extensional pluton during decompression and cooling are demonstrated for a metamorphic core complex in central turkey new geochronologic data for the nigde massif of the central anatolian crystalline complex document an interval of melting crystallization and pb loss at elevated ambient temperature 85 92 ma determined by u pb shrimp within a single sample of peraluminous granite crustal melting was syn to post peak metamorphism 725 c dated at 91 0 2 0 ma by shrimp analysis of zircon rims in sillimanite schist that is part of the country rock to the pluton metamorphic monazite in sillimanite schist yields a younger age of 84 7 0 7 ma for matrix grains and inclusions in garnet hornblende 40ar 39ar ages from amphibolites interlayered with the sillimanite schists coincide with the timing of emplacement of magma into metamorphic country rock that had experienced cooling and decompression to 12 km and 550 c the similarity of 40ar 39ar ages for biotite in the pluton 79 ma and hornblende 79 81 ma biotite 74 76 ma and muscovite 78 83 ma in the metamorphic country rock document rapid cooling 100 c my following intrusion the coincidence of the youngest u pb magmatic zircon ages and the oldest 40ar 39ar ages combined with structural evidence for normal sense shearing and tectonic unroofing in the late cretaceous suggests that intrusion was driven by extension we propose that crustal melting occurred in the middle crust during prograde metamorphism and or initial decompression and that ascent and emplacement of magma as a pluton were only triggered once extension occurred and removed upper crustal rocks

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

gravity driven continental overflow and archaean tectonics whether modern tectonic processes differ substantially from those in archaean times 2 500 myr ago remains controversial one view1 is that archaean tectonic processes were some combination of modern ones occurring faster or more shallowly because of the larger heat output of the early earth but others2 have proposed that significantly different processes operated here i argue that gravitational spreading of archaean continents would have caused them continuously and pervasively to overflow onto adjacent ocean basins and that this process would have naturally ceased at the end of the archaean era because modern continental crust is believed to be ductile rather than brittle below a depth corresponding to a temperature of about 350 400 c ref 3 it seems likely that such a ductile zone was universally present within the hotter archaean continental crust if the mean geothermal gradient of the continents had exceeded 25 30 c km 1 then the resulting ductile zone would have caused continental overflow to occur and such a process can account for many of the distinctive peculiarities observed in the archaean geological record the cessation of continental overflow corresponds naturally to the stabilizing cratonization which marked the end of the archaean era with its timing dependent on the evolution of both the geothermal gradient in the continents and the depth of the ocean basins

pervasive melt migration from migmatites to leucogranite in the shuswap metamorphic core complex canada control of regional deformation abstract the shuswap metamorphic core complex exhumed in the hinterland of the canadian cordillera displays a 15 km structural section from migmatites to leucogranite it comprises a lower unit dominated by anatectic migmatites exposed in the core of domes at higher structural level the transition from a middle amphibolite facies unit to an upper crustal unit is marked by a low angle detachment associated with the emplacement of laccoliths of leucogranite the amphibolite facies middle unit is permeated by a network of granitic veins feeding the laccoliths and structurally connected to the migmatites of the lower unit this paper describes the geometric textural and structural characteristics and the distribution of the granitic fraction at the outcrop scale in the amphibolite facies unit separating the migmatites from the leucogranite this analysis constrains the mechanisms of melt migration within a plurikilometer scale section across high grade rocks of the middle crust in fertile lithologies metapelites felsic amphibolites the granitic fraction in part generated in situ forms a diffuse network of concordant veins feeding discordant veins and structural sites such as the foliation shear zones and boudin necks discordant granitic veins are oriented perpendicular and to a lesser extent parallel to the mineral and stretching lineation in refractory lithologies the granitic fraction is dominantly intrusive and displays sharp contacts with the host rock however distribution of the granitic fraction shows the same characteristics as for fertile lithologies the melt fraction migrated through a network of fractures taking advantage of mechanical weaknesses such as the foliation plane or forming dikes oriented dominantly perpendicular to the regional stretching and mineral lineations these geometric and textural characteristics suggest that granite migration in these rocks is achieved dominantly by viscous flow of the melt solid through the solid matrix driven by its buoyancy and controlled by mechanical anisotropy of the rock and local dilation created by heterogeneous deformation the relationship between the regional fabric and the distribution of the granitic fraction indicates that regional deformation incremental and finite played a major role in providing pathways for melt migration the formation of laccoliths of leucogranite at higher structural levels suggests that upward melt migration led to accumulation of granitic magmas these observations are consistent with a model of pervasive melt migration through a network of interconnected concordant and discordant granitic veins feeding laccolithic plutons extracted from the dominantly solid matrix during vertical thinning and compaction at a regional scale the presence of both transposed and intact granitic networks indicate an overlap between deformation and granite migration as a continuum or in pulses the methodology and terminology proposed in this paper provide some insights on the behavior of the granitic fraction in the middle crust this approach should be applicable to other high grade terrains exhumed in the cores of orogenic belts

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

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

magma emplacement in transpression the santa olalla igneous complex ossa morena zone sw iberia the santa olalla igneous complex a late variscan group of intrusions located in the ossa morena zone sw iberia has been the focus of a gravity and structural study the structure outlined by the foliation map is complex showing two different structural domains one characterized by vertical and the other by horizontal magmatic foliations the vertical fabrics are restricted to the ne half of the complex which is in direct contact with a variscan sinistral strike slip fault cherneca fault whereas the horizontal fabrics are developed in the sw half of the complex which is characterized by a horizontal tabular geometry gravity modeling indicates that the deeper floor of the plutons is closely related to the ne margin and the cherneca fault an emplacement and structural evolution model for this igneous complex is proposed following these structural and gravity results 1 magma ascent was favored by releasing bends in the trace of the cherneca fault 2 when magma reached the present level it intruded to the sw with a horizontal sheet geometry generating the subhorizontal foliation domain 3 after emplacement the ne half of the complex suffered the external tectonic stress field provoked by sinistral motion along the cherneca fault subsequently generating the subvertical magmatic foliation domain

mechanical consequences of granite emplacement during high t low p metamorphism and the origin of anticlockwise pt paths high temperature t low pressure p metamorphic belts preserve evidence of metamorphism under extremely perturbed and hence transient thermal regimes pt paths are commonly anticlockwise that is the maximumt was attained prior to or at the same time as the maximump with cooling at near constant or increasingp and in many terrains peak metamorphic temperatures prevailed during convergent deformation using a simple coupled thermal mechanical model that assumes a strongly temperature dependent rheology for the continental lithosphere we show that the coincidence of metamorphism with convergent deformation in high t low p terrains may reflect the thermal weakening attendant with rapid advective movement of heat within the lithosphere via granite magma ascent the model shows that the magnitude of the thermal weakening effect is sensitive to the critical temperature for granite segregation tm crit the depth of emplacement of the magmas and most importantly the granite forming mechanism during the prograde cycle material points attain maximum temperatures during a transient 3 ma high strain rate pulse withe up to 10 14 s 1 at levels near the site of granite emplacement the attainment of maximum pressures follows peak temperatures with rapid cooling paths that are close to isobaric or involve slight compression

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

near orthogonal deformation successions in the poly deformed paleoproterozoic martimo belt implications for the tectonic evolution of northern fennoscandia abstract structural mapping integrated with aeromagnetic and anisotropy of magnetic susceptibility ams data form complimentary and robust tools for analysis of regional structures and fabric overprinting relationships this multi scale approach is used to constrain the geometry and tectonic evolution of sparsely exposed and flat lying outcrops of the martimo belt mb of northern fennoscandia the mb records a history of near orthogonal deformations upper greenschist to lower amphibolite facies metamorphism between ca 1 92 1 79 ga the earliest deformation d1 involved east directed thin skin thrusting and the development of recumbent f1 folds and a n s trending s1 fabric d2 deformation is prominent in the central part of the mb and is characterized by n s shortening with the development of e w trending f2 folds an associated pervasive steeply dipping s2 foliation and type 2 fold interference pattern the mb and mellajoki suite exhibit regional scale d2 synformal and antiformal refolded structures respectively d3 is more heterogeneously developed and manifests bimodal orientations with nnw sse and wnw ese structural trends it is pronounced in the central and eastern parts of the mb but weakly developed in the western part the steep dip of s3 in the region suggests that it formed in response to ne sw bulk shortening d4 structures have a conspicuous nne ssw trend that is visible as lineaments on the aeromagnetic image and locally in the field the last deformation event d5 affected almost the entire mb and was characterized by broadly wsw ene oriented crustal shortening the n s oval shape geometry of the eastern part of the mb is attributed to the d5 event contrasting with well developed overprinting relationships between d1 and d3 structures the relative timing of d3 d5 could not be established in the study area but can be inferred from structures with similar trends in the central lapland granitoid complex which is located north of the mb the d1 d5 deformation scheme can be readily reconciled with the tectonic evolution of the lapland savo fennian svecobaltic and nordic orogens of the composite svecofennian orogen the age of d1 in the northern part of the lapland savo orogen is 1 91 ga based on crosscutting granitoids and maximum deposition ages of sedimentary rocks d2 1 90 1 89 ga has been linked to orogen parallel shortening during the late stage of southern part of the lapland savo orogen several orthogonal tectonic events in northern fennoscandia indicate amalgamation of crustal blocks around the core of the archean karelia continent at 1 92 1 77 ga and that fennoscandia was a key component of the columbia hudsonia nuna supercontinent

spatial distribution of melt bearing structures in anatectic rocks from southern brittany france implications for melt transfer at grain to orogen scale abstract in this study we present quantitative spatial information on the one and two dimensional distribution of inferred melt bearing structures in anatectic supracrustal rocks of the southern brittany migmatite belt south of the transcurrent south armorican shear zone sasz based on these data we infer the mechanism of melt extraction from partially molten crust former melt bearing structures include foliation parallel leucosomes and cross cutting granitic leucosomes that infill inter boudin partitions and extensional shear surfaces as well as discordant dykes of granite petrographic i e mineralogical and microstructural continuity of granite from structure to structure suggests that they once formed a continuous melt bearing network measurements along one dimensional line traverses perpendicular to layering of stromatic migmatite exposed in clean sub horizontal outcrop surfaces provide information about thickness and spacing distributions of foliation parallel leucosomes most leucosome thicknesses fall in the range of 1 10 mm with upper limits around 20 30 mm the number of thicker layers decreases abruptly with increasing thickness which is inconsistent with scale invariance this suggests that leucosome formation was controlled by short range melt movement along grain boundaries to form melt rich layers constrained by pre existing compositional layering spacing distributions also are not scale invariant however the large percentage of leucosomes 40 60 in these line traverses suggests that spacing distributions may be controlled in part by impingement of leucosomes making it difficult to derive genetic information from these data qualitative observation of inferred melt bearing structures in mutually perpendicular two dimensional exposures from the same outcrop reveals anisotropy of the leucosome network related to a well developed sub horizontal quartz feldspar lineation reflecting stretching associated with transcurrent movement along the sasz analysis of these two dimensional distributions using the box counting method corroborates the observed anisotropy but indicates that leucosome morphology and perhaps distribution is not scale invariant the applicability of the box counting method or of fractal analysis to understanding melt movement in migmatites is discussed in light of these results based on the anisotropy of melt bearing structures we infer that melt movement in structures now represented by layer parallel leucosomes was primarily sub horizontal these layers fed steeply dipping structures now represented by cross cutting leucosomes in particular those developed at inter boudin partitions and granite dykes the formation and orientation of these steeply dipping structures was in part controlled by far field stresses related to dextral displacement along the sasz melt extraction is inferred to have occurred along these steeply dipping structures extracted melt accumulated in plutons at higher crustal levels such as the quiberon sarzeau and guerande granites

evolution of ptygmatic folds in migmatites from the type area s finland abstract it is proposed that ptygmatic folds in migmatites form during leucosome melanosome differentiation volume loss by depletion of quartz and feldspar takes place in the interlimb domains of folds leaving a residual melanosome buckling leucosomes increase in volume due to this process and are protected from substantial thinning concurrent with tectonic segregation albeit at a lower rate the leucosome expands by replacing the melanosome this process is induced by fluid activity at the leucosome boundary as suggested by the occurrence of secondary offshoot veinlets segregation and growth causes the most typical aspect of ptygmatic folds preservation of an overall buckle shape during the stage when the folds become isoclinal and devoid of interlimb matrix

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

a cleavage triple point and its meso scopic structures the mustio sink svecofennides of sw finland abstract a cleavage triple point ctp structure is analyzed located at the west side of the mustio gneiss dome in the svecofennides of southwest finland the presence of the ctp and the pattern of mesoscopic fold structures exclude the origin of the mustio dome by successive interference of fold phases the highly variable deformation structures are explained in a single phase deformation model by using the theoretical specific strain environments of a ctp these environments are 1 horizontal oblation on top of a dome 2 transition from horizontal to vertical oblation on the flanks of a dome and 3 vertical constriction in the center of the ctp it is shown that each strain environment is associated with specific development of foliation folds mesoscopic fold interference and strain intensity the theoretical strain environments are confirmed by strain analysis

the mode of deformation in the orogenic mid crust revealed by seismic attribute analysis 1 the processes and the structures within the middle and the lower crust are intimately related to the evolution of orogenies but more detailed knowledge is needed of these deep crustal processes seismic reflection data chiefly displayed in amplitude are commonly used to interpret deep crustal structures the strongest amplitude events are probably related to lithological changes and do not necessarily correspond to geological structures tectonic boundaries or deformation fabrics furthermore the detailed structures and the strain distribution between the interpreted structural boundaries remain obscure here we show an example of how seismic attributes combined with the seismic facies interpretation technique can be used to enhance 2d seismic reflection data from the palaeoproterozoic svecofennian crust of southern finland to reveal unprecedentedly detailed information about the deformation fabrics within the mid crust of a collisional orogen the images are plausibly interpreted to show that the extension lateral flow of the orogenic middle and lower crust was mainly accommodated by kilometer scale s c structures the structures form penetrative deformation fabrics which are correlated with outcrop observations the successful enhancement of the seismic data confirms the ductile extension affecting hot orogenic crust and gives new information about the strain distribution of the regional syn to late orogenic deformation the seismic attribute method and the seismic facies interpretational approach described in this paper should be applicable to other seismic datasets from the crystalline basement

brittle reactivation of ductile precursor structures the role of incomplete structural transposition at a nuclear waste disposal site olkiluoto finland abstract reactivation of discrete deformation zones that are orientated favourably with respect to the stress field is a well known phenomenon what is less clear is the role of other structural features and heterogeneities in localizing deformation in this paper we describe how brittle deformation structures are localized into zones of incomplete structural transposition inherited from earlier ductile deformation phases in our example these zones of incomplete structural transposition are characterised by localised high strain structures of the latest ductile deformation stage including short limbs of strongly asymmetric folds and anastomosing networks of minor shear fabrics when such zones are systematically organized and orientated favourable with respect to the stress field they can be very efficient in localizing deformation and forming new fault zones applied to the site of the planned geological repository of nuclear waste in olkiluoto finland the recognized structural inheritance provides tools to understand the geometries networks and kinematics of the brittle fault zones and the related secondary fracturing which together define the rock mechanical and hydrogeological framework for the repository

tectonic pumping of pervasive granitic melts finite element 2d models are used to study how tectonic stresses pump pervasive granitic melts within migmatites we start by assuming elliptical melt filled veins of different orientations interconnected by the assumption that they share the same pressure the melt is then redistributed in the vein array by the application of lateral compression or extension the effective permeability of vein networks is thought to be up to 13 orders of magnitude higher than the matrix permeability of 10 19 m2 this allows us to approximate pressure equilibration and neglect irreversible compaction in the matrix this simple configuration allows solution of constitutive equations for the equation of state in the viscoelastic matrix the melt in the vein array and the mechanical equilibrium along with force balance on the walls of the deforming veins a rectangular box is pressurized by a constant load applied to one side and continuously deformed by shift of another side with constant velocity new veins cannot initiate and pre existing veins cannot migrate bodily down pressure gradients however potential flaws are cut in line with the tip of every pre defined vein this means that veins not only close open and shear as they rotate but can also undergo limited propagation at rates dictated by the bulk deformation of the matrix we use constant viscosities s in the range 1017 1018 pa s deformation rates e vary between 10 10 and 10 9 s 1 and a total strain up to 4 5 isolated veins parallel and normal to 1 have melt pressures between p0 and p0 4 se the mean vein pressure differs from these extremes and is equilibrated by driving melt from shrinking veins into veins parallel to 1 which widen and lengthen the above assumptions result in an asymptotic pattern of stress distribution and the amount of melt redistributed at given strain approaching the kinematic limit with time this occurs whatever the viscosity of the matrix and the strain rate multilayered systems are modeled by pre defining a single vertical vein crossing the planar horizontal boundary between two uniform media compression parallel to the layering expels melt from the part of the vein in the more viscous layer to open and extend that part of the same vein in the less viscous layer melt moves in the opposite direction during lateral extension if sheet like bodies become sufficiently tall to become buoyant horizontal sheets resulting from lateral compression are likely to rise to higher crustal levels as diapirs while vertical sheets resulting from lateral extension are more likely to ascend as dikes

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

the evolution of the mustio gneiss dome svecofennides of sw finland abstract the mustio gneiss dome situated in the 1 92 1 7 ga svecofennides of sw finland is investigated the dome formed relatively late in the tectono metamorphic history of a multiple deformed schist and gneiss terrain micro meso and macrostructures are described and related to metamorphic development evidence is presented for an early f1 phase which produced large scale recumbent folds and a thickened tectonostratigraphy characterized by a layer parallel penetrative schistosity most conspicuous however are upright tight to isoclinal f2 folds with w e striking axial planes f2 is cross folded by f3 which is characterized by open folding with ne running axial planes the dome has a migmatite core and has been formed along the crest of a regional f2 antiform both f3 cross folding and diparism are consistent with the structural data and it is suggested that the dome was formed by a combination of these mechanisms regional implications and relationship with mantled gneiss domes in general is briefly discussed

facies analysis of a 1 9 ga continental margin back arc felsic caldera province with diverse zn pb ag cu au sulfide and fe oxide deposits bergslagen region sweden the bergslagen mining district in south central sweden has been a major metal producer for more than 1 000 years in geologic terms bergslagen is the intensely mineralized part of a large early proterozoic mainly 1 90 1 87 ga felsic magmatic region of mainly medium to high metamorphic grade in the baltic shield the district contains a diverse range of ore deposit types including banded iron formation magnetitecalc silicate skarn manganiferous skarn and carbonate hosted iron ore apatite bearing iron ore stratiform and strata bound zn pb ag cu au sulfide ores and w skarn most ore deposits occur in hydrothermally altered metavolcanic rocks and associated metalimestones and skarns this study is a field oriented regional facies analysis based on recognition of primary volcanic and sedimentary features through the strong overprints of hydrothermal alteration deformation and metamorphism the region is interpreted as an extensional probably back arc active continental margin magmatic region it evolved through stages of intense magmatism thermal doming and crustal extension followed by waning extension waning volcanism thermal subsidence reversal from extension to compressional deformation regional metamorphism and structural inversion the volcanic successions comprise interfingering proximal near vent medial volcano flanks and distal volcano margin facies associations of rhyolitic pyroclastic caldera volcanoes and subordinate dacite rhyolite complexes nonwelded to poorly welded pyroclastic flow and fallout units and their rapidly resedimented subaerial and subaqueous equivalents are the most abundant volcanic facies subvolcanic porphyritic intrusions and cryptodomes are also abundant the pyroclastic debris was shed from mainly shallow water and subaerial proximal areas to shallow and moderately deep water above and below storm wave base distal areas most ore deposits formed during the regional waning volcanic stage and occur in medial to distal facies associations that comprise rhyolitic ash siltstone limestone and vitric crystal sandstone and breccia these mineralized facies associations were deposited in subaqueous environments mainly below the fair weather wave base but locally above the storm wave base water depths are interpreted to have been mainly 10 to 500 m with local deeper areas although the ore deposits occur in medial to distal facies associations many occur above or laterally adjacent to subsided volcanic vent complexes this suggests a spatial temporal and possible genetic relationship between several ore deposit types and the late magmatic stage of these volcanic centers except for the apatite bearing iron deposits it is uncertain if the ores are genetically related to specific magmatic hydrothermal events from specific volcanoes this is possible especially for the base metal sulfide deposits which have intense localized footwall alteration in felsic volcanic rocks however an important role of the volcanic centers may have been to provide strong positive pertubations or magmatic hydrothermal input to the regional convective geothermal system the main zn pb ag cu au ore deposits span a range between two end member types stratiform ash siltstone hosted sea floor deposits that have similarities to some stratiform sediment hosted zn pb ag deposits especially the broken hill type deposits and strata bound volcanic associated limestone skarn subsea floor replacement deposits that more closely resemble felsic volcanic associated massive sulfide deposits facies models are presented for both the banded iron formations are interpreted as chemical suspension deposits rhythmically interbedded with rhyolitic ash siltstone the skarn iron ores are strata bound in limestones and include metamorphosed sedimentary ores and metasomatic replacements apatite bearing iron ores differ in setting from the other ores in being formed within a dacitic intrusive complex that may be the root zone of a dacite rhyolite volcanic center

age and structural constraints on the tectonic evolution of the paleoproterozoic central lapland granitoid complex in the fennoscandian shield abstract the paleoproterozoic central lapland granitoid complex clgc forms the northern core of the fennoscandian shield we present here new u pb age data and review the existing ones from plutonic and metasedimentary rocks of the clgc and integrate the results with the structural field observations to decipher the tectonic history of this region granitoids vary in age from 2 43 ga to 1 78 ga whereas metamorphic zircon overgrowths both in granites and metasedimentary rocks vary between 1 90 ga and 1 76 ga with several age peaks indicating prominent metamorphic pulses the almost continuous growth of metamorphic zircon domains favors that the central part of the clgc was near solidus for 100 120 ma a succession of five deformations and associated foliation producing events which were near orthogonal to each other have been identified east vergent thrusting and stacking d1 commenced at ca 1 92 ga and led to the onset of barrovian type metamorphism followed by a radical tectonic shift that yielded e w trending d2 structures exhumation at 1 88 1 87 ga caused decompression melting of thermally evolved thickened crust and led to the emplacement of large volumes of granites and triggering of buchan type metamorphic conditions ne sw collision d3 at the southwestern margin of fennoscandia heterogeneously overprinted the s3 fabric on d2 structures orogen parallel nnw sse buckling at 1 87 1 85 ga formed oroclines in the hot linear arc segments renewed nnw sse shortening d4 at 1 84 1 81 ga produced long wavelength folds and caused the formation of domal structure in the clgc the final 1 78 1 76 ga far field collision d5 in the west overprinted all the pre existing structures this complex multi directional tectonic scenario provoked the intermittent exhumation phases between different contraction events of the clgc

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

evolution of a svecofennian mantled gneiss dome in sw finland with evidence for thrusting abstract the structure and petrology of a dome in svecofennian migmatitic gneisses schists amphibolites and granitoids on kemlo island is described the structure resembles classical mantled gneiss domes in that it comprises a core and mantle but differs in that intrusive granitoids occur in the mantle rather than the core four generations of folds are recognised and structural and petrological evidence is presented for an f 2 related thrusting origin for the discontinuity that separates core and mantle the metamorphic evidence suggests that the core was subject to low pressure amphibolite facies conditions at a time prior to thrusting when the mantle was subject to higher pressure amphibolite facies conditions it is pointed out that metamorphic jumps elsewhere on kemio can be better explained by thrusting than by normal faulting as previously suggested the structure is shown to be adequately explained as an interference structure resulting from superposition of f 3 and f 4 folding the possibility of diapirism cannot be eliminated but there is no necessity to invoke such a process and no evidence for it it is suggested that the interference pattern interpretation plus a thrusting origin for the core mantle contact and the folded foliation may have more general applicability the structural and metamorphic history is very similar to that of phanerozoic mountain belts that are interpreted as a product of plate tectonics it is suggested that the same model can be applied to the svecofennian