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

impact of atmospheric forcing on antarctic continental shelf water masses the antarctic continental shelf seas feature a bimodal distribution of water mass temperature with the amundsen and bellingshausen seas flooded by circumpolar deep water that is several degrees celsius warmer than the cold shelf waters prevalent in the weddell and ross seas this bimodal distribution could be caused by differences in atmospheric forcing ocean dynamics ocean and ice feedbacks or some combination of these factors in this study a highly simplified coupled sea ice mixed layer model is developed to investigate the physical processes controlling this situation under regional atmospheric forcings and parameter choices the 10 yr simulations demonstrate a complete destratification of the weddell sea water column in winter forming cold relatively saline shelf waters while the amundsen sea winter mixed layer remains shallower allowing a layer of deep warm water to persist applying the weddell atmospheric forcing to the amundsen sea model destratifies the water column after two years and applying the amundsen forcing to the weddell sea model results in a shallower steady state winter mixed layer that no longer destratifies the water column this suggests that the regional difference in atmospheric forcings alone is sufficient to account for the bimodal distribution in antarctic shelf sea temperatures the model prediction of mixed layer depth is most sensitive to the air temperature forcing but a switch in all forcings is required to prevent destratification of the weddell sea water column

ocean circulation its effects on seasonal sea ice simulations a diagnostic ice ocean model of the arctic greenland and norwegian seas is constructed and used to examine the role of ocean circulation in seasonal sea ice simulations the model includes lateral ice motion and three dimensional ocean circulation the ocean portion of the model is weakly forced by observed temperature and salinity data simulation results show that including modeled ocean circulation in seasonal sea ice simulations substantially improves the predicted ice drift and ice margin location simulations that do not include lateral ocean movment predict a much less realistic ice edge

studies of the arctic ice cover and upper ocean with a coupled ice ocean model an ice ocean model has been developed by coupling the hibler ice model to a three dimensional ocean model that consists of a turbulent mixed layer model and an inverse geostrophic model the coupled model has a horizontal grid spacing of 127 km and has 17 vertical levels extending to the ocean bottom the model is forced with 12 hourly general circulation model derived atmospheric fluxes for the year 1986 for which good quality ice edge analyses and buoy tracks were available for comparisons the results are presented for the fifth year of a repetitive simulation with the 1986 fluxes at which time the system has reached a statistical equilibrium the seasonal and geographical variations of the ice cover and the upper few hundred meters of the ocean have been examined including the heat and salt budgets the computed heat fluxes and mixed layer depths mlds fall in the observed seasonal ranges with winter heat fluxes ranging from 15 w m 2 in the central arctic to about 500 w m 2 in the barents sea area and summer fluxes from about 5 w m 2 under the ice to about 30 w m 2 in the various marginal ice zone miz edge areas the corresponding winter mlds are found to be about 25 75 m in the central arctic to a deep 800 m in the greenland sea typical summer mlds are between 5 and 30 m in all seasons the miz was found to be the center of flux activity for both heat and salt with the processes of advection diffusion atmospheric forcing and vertical oceanic fluxes having their largest values here and of comparable magnitudes values of the heat flux components in the miz exceed those found under the ice by an order of magnitude or more and those in the open water region by a factor of 2 for salt the situation is similar except in the summer june through september when a lot of salt flux activity takes place under the ice comparisons are made with naval polar oceanography center npoc analyses for ice concentration and ice edge and with observed arctic buoy tracks in the same 1986 time period the computed ice edge positions have comparable accuracy to previous three dimensional coupled ice ocean studies with too much ice growth during the winter in the barents sea and too little ice east of greenland the ice thickness distributions however are much better with a monotonic increase of the ice thickness from the siberian coast east toward the canadian archipelago with maximum winter values of about 5 6 m

the 1 75 1 68ga anorthosite mangerite alkali granitoid rapakivi granite suite from the northern north china craton magmatism related to a paleoproterozoic orogen abstract the amgrs is a suite of anorthosite mangerite alkali granitoid rapakivi granite in the northern north china craton ncc new zircon shrimp and laser ablation inductively coupled plasma mass spectrometry la icpms u pb dating of the damiao anorthosite changsaoying k feldspar granite lanying anorthosite and quartz syenite and the gubeikou k feldspar granite yielded emplacement ages of 1726 9 ma 1753 23 ma 1739 43 ma 1712 15 ma and 1692 19 ma respectively combined with previous geochronological data from the damiao norite and mangerite and shachang rapakivi granite it indicates that the whole amgrs was intruded between 1750 ma and 1680 ma the damiao anorthosites and norites exhibit a high mg and this decreases from mafic up to 63 to felsic t dm c ages ranging from 2 32 ga to 2 43 ga and 2 70 ga to 2 88 ga respectively this suggests that these rocks are the fractional crystallization products of one parental magma zircon hf and whole rock sr nd pb isotopic systematics combined with high la yb 10 and intermediate th ta ratios suggest that the parental magma of the amgrs could have been derived from em i lithosphere mantle enriched by recycled archean continent crustal materials accompanied by some assimilation of lower crustal components the mafic to felsic compositions of the amgrs are different from typical bimodal rocks formed in an anorogenic intra continent rift setting but resemble those emplaced during post collisional post orogenic extensional tectonic setting this is compatible with their age of

development of a simple upper ocean and sea lce model abstract results are described for three simple numerical models of the upper ocean and sea ice with prescribed atmospheric forcing the ability of each model version to simulate the observed sea surface temperature sst is assessed as a basis of comparison for future coupled experiments with atmospheric gcm s lie upper ocean model versions raw from a slab of fixed thickness to a variable depth mixed layer above a variable exponential temperature gradient representing the seasonal thermocline sea ice thickness is determined thermodynamically by local melting or accretion and the effects of ice transport and leads are neglected each version is tested by integrating to equilibrium with horizontal advection neglected using monthly climatological atmospheric data for a selected north south section in the mid pacific ocean and the results for different model versions are compared with each other and with available observations it is found that the fixed slab version gives realistic sea ice thickness a

integrated seismic studies of the baltic shield using data in the gulf of bothnia region in the autumn of 1989 a co operative experiment involving 12 research institutions in northwestern europe collected 2268 km of deep seismic reflection profiles in the gulf of bothnia and the baltic

migratory tectonic switching western svecofennian orogen central sweden constraints from u pb zircon and titanite geochronology abstract the forsmark area in the western part of the svecofennian orogen central sweden is situated between two major palaeoproterozoic tectonic domains that show contrasting histories with respect to timing of igneous activity ductile deformation and metamorphism geological features common to both the adjacent domains are found at forsmark which consequently is a key area for understanding the tectonic evolution of the svecofennian orogen in this region new sims and tims geochronological data from several samples in a limited area at forsmark constrain the timing of igneous activity ductile deformation and metamorphism in this area u pb zircon dating in combination with field studies reveals the existence of two calk alkaline igneous suites at forsmark the older and most voluminous plutonic suite intruded at 1 89 1 87 ga and is affected by penetrative yet variably intense ductile deformation with the development of a ductile grain shape fabric under amphibolite facies metamorphic conditions and folding the younger and less voluminous hypabyssal suite intruded at 1 86 1 85 ga during the waning stages of this penetrative deformational phase furthermore u pb titanite dating shows that the area has been affected by one or more tectonothermal events after regional fabric development and possibly even after 1 83 ga the deformation during these events involved more restricted strain along discrete zones the forsmark data in combination with a compilation and evaluation of available geochronological data from the time interval 1 91 1 84 ga in central sweden points to the existence of at least two major tectonic cycles in this region each cycle is characterised by igneous activity associated at least partly with extension a short interval of compressional deformation and migration of the tectonic activity in this paper we discuss two highly contrasting tectonic models that may explain the cyclic tectonic evolution of the western part of the svecofennian orogen in central sweden the favoured model involves continuous subduction with a constant polarity beneath a single active continental margin combined with alternating subduction hinge retreat and advance the model involves migration of what has been described as tectonic switching in the younger accretionary orogenic systems of eastern australia palaeozoic lachlan orogen and new zealand

rapakivi granites and related rocks an introduction abstract this article is an introductory chapter for the proceedings of the last seventh meeting of the international geological correlation programme igcp project 315 correlation of rapakivi granites and related rocks on a global scale 1991 1996 based on current knowledge progress made during the six years of the project and the seven papers published in this special issue a synthesis of the age distribution geotectonic environment petrogenesis geochemistry and metallogeny of the rapakivi granites and related rocks is presented

comparison of crustal structure along dss profiles in se fennoscandia summary main features of the crustal structure of the karelian and svecofennian provinces are discussed on the basis of data obtained from deep seismic sounding dss profiles across the se fennoscandian shield the data show that in the ancient karelian province the crust is thinner about 40 km the average velocity is lower about 6 4kms l and the moho boundary is a sharp discontinuity which produces strong reflections the high velocity layer in the lower crust if it exists at all is thin in the younger svecofennian province crustal thickness averages about 45 km with a velocity of 6 6 6 8 km s the reflectivity of the moho is usually weak and a high velocity layer with velocities greater than 7 km s is ubiquitous in the lower crust high velocities in the lower crust and upper mantle are found in deep moho areas beneath the younger granitic intrusions gothian complex and vyborg rapakivi granites the crust is thinner than on average

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

evidence for 1 82 ga transpressive shearing in a 1 85 ga granitoid in central sweden implications for the regional evolution abstract two crustal scale shear zone systems the storsjon edsbyn deformation zone and the hassela shear zone join in the western part of central sweden and form an anastomosing pattern of ductile shear zones several of these up to 1 km wide zones truncate an intrusion previously defined as a component of the major ca 1 80 1 77 ga magmatic suite the revsund granitoids in north central sweden however within the southern part of the intrusion titanites that are components of the fabric in a pervasively deformed shear zone forsaan have a u pb tims age of 1816 2 ma zircon porphyroclasts in the same fabric yield a u pb sims age of 1849 14 ma overlapping with the u pb tims age of 1851 2 ma for titanites from an undeformed granitoid these results are interpreted to define the ages of ductile shearing and magma emplacement respectively this interpretation is supported by microstructures as well as the regional structural pattern characterised by shear zones enveloping virtually undeformed pods of the granitoid previously reported and locally existing magmatic flow structures indicate some syn emplacement deformation the kinematic results characterised by a high proportion of pure shear within the granitoid and oblique dextral shearing along its margin reflect strain partitioning under transpressive conditions the kinematic and geochronological results fit into the regional framework of late orogenic localised deformation along crustal scale shear zones recent absolute datings of shear fabrics defines ca 1 82 1 80 ga as a period of shear zone activity in central sweden temporally this period overlaps with the emplacement of s type granites and granitoids derived from deeper crustal levels a similar situation exists in southern finland structural geochronological and geophysical data suggest that the crustal scale shear zones in central sweden and southern finland may be related temporally and spatially the age and the post tectonic nature on a regional scale of the dated 1 85 ga granitoid have important regional implications as both the age of the intrusion and the ductile shearing are older than the established age range 1 80 1 77 ga of the revsund granitoids either an extended intrusive history for the suite is indicated or the granitoids in this part of the type areas should be excluded from the revsund suite their post tectonic nature implies that the pervasive svecokarelian deformation in the area occurred earlier than generally assumed 1 85 1 80 ga on the other hand approximately coeval calc alkaline granitoids to the south east of the investigated area are folded and foliated pervasively indicating a later svecokarelian evolution in that area i e that the granitoids are located in domains with separate orogenic evolutions

reflectivity and seismic properties of the deep continental crust the recovery of drill core from a section of upper amphibolite facies terrane located in the inner piedmont of south carolina provides a rare opportunity to study in detail the seismic character of rocks which were once part of the deep continental crust seismic reflection profiles over the inner piedmont in the vicinity of the drill hole reveal subhorizontal high amplitude reflections the origin of these reflections has been investigated by measuring compressional wave velocities as a function of confining pressure and densities in 150 samples obtained from 305 m of drill core synthetic seismograms generated from the velocity and density measurements demonstrate that reflections originate from subhorizontal metamotphic layering probably originating from lower crustal ductile flow fine tuning of the metamotphic layering which is temperature and pressure related can produce bright spots in the deep crust anisotropy resulting from preferred mineral orientation is another important propeny of this sectim velocities are low for vertical wave propagatim whereas horizontal velocities are relatively high horizontal velocities show mly a slight variatim with azimuth which is unlikely to be detected using standard refractim techniques reflectivity decreases with increasing anisotropy critical to this study has been the continuous coring of a major section of high grade metamotphic terrane which has provided the necessary stratigraphic and lithologic cmtrol for the generation of accurate synthetic seismograms

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

petrology and geochemistry of mafic granulite xenoliths from the lahtojoki kimberlite pipe eastern finland abstract the lahtojoki kimberlite pipe in kaavi eastern finland contains lower crustal mafic granulite xenoliths with a mineral assemblage cpx amph pl grt opx bt the tweequ thermobarometry indicates crystallization at ca 800 900 c and 0 75 1 25 gpa which corresponds with crustal depths of ca 22 38 km reaction textures and results of thermobarometry in some xenoliths indicate post peak decompression with cooling chemical composition of the xenoliths suggest that their protoliths crystallized from basaltic melts with some k enrichment their ree patterns are slightly lree enriched some xenoliths having small negative eu anomalies the u pb ion probe dating of zircons from two samples yielded variable ages for zircons even in the same xenolith between ca 2 6 and 1 7 ga these ages correspond with main late archaean and palaeoproterozoic orogenic events in the fennoscandian shield the sm nd garnet clinopyroxene isochron age from one dated sample is 1 6 ga which represents either a cooling or reheating of the lower crust by rapakivi magmatism because the petrology geochemistry and ages of xenoliths do not correlate well with the nearest exposed archaean mafic granulites it is evident that the present lower crust underlying the archaean rocks has a considerable palaeoproterozoic component

seismic reflectivity of mylonite zones in the crust deep fault zones are characterized by mylonitic rocks with strong preferred orientation of constituent minerals and retrograde mineral assemblages sonic velocities are consequently lower for propagation directions normal to mylonite foliation than in surrounding unmylonitized rocks synthetic seismograms computed for low velocity near horizontal thick mylonite zones of complex laminated geometry show multicyclic reflections with amplitudes up to twice those generated by a single interface seismic characteristics of mylonite zones are sufficient to produce reflections in crystalline rocks provided noise levels are relatively low

origin of reflections from the brevard fault zone the brevard fault zone is a belt of mylonitic metamorphic rocks that extends northeast over 700 km from alabama into virginia seismic reflection studies show that the fault zone produces excellent reflections which dip gently to the southeast and sole at depth the origin of these reflections has been investigated by measuring compressional wave velocities to 800 mpa confining pressure in 127 samples obtained from drill core which continuously sampled the fault zone to a depth of over 300 m densities are reported as well as chemical analyses at selected depths anisotropy at elevated pressures ranges within the fault zone from less than 5 to over 26 similarly the fault zone rocks show significant chemical variability with sio2 ranging from 58 0 to 70 4 by weight synthetic seismograms generated from the velocity and density measurements show that the reflections originate within the brevard fault zone from the complex interaction of compositional variations and seismic anisotropy resulting from ductile strain essential to this study has been the continuous coring of a major section of the brevard fault zone which has provided the necessary stratigraphic control as well as unweathered samples for laboratory studies

a note on chimney formation in ice edge regions wind driven ice ocean interaction in the marginal ice zone has been shown to contribute to enhanced entrainment when the winds are favorable for upwelling at the ice edge if a strong surface heat loss is present formation of a deep convective region is possible resulting from an ever decreasing buoyancy and a rapid deepening of the mixed layer i e a chimney this chimney does not extend to the bottom but loss of buoyancy would indicate eventual overturning to the bottom in the studied two layer ocean model where initially a fresh and cold freezing mixed layer lies above a stagnant warm and saline lower layer a catalyst for the convection is the ice and especially the existence of the ice edge which has both a dynamic and thermodynamic interaction with the ocean in the coupled ice ocean model the entrainment between the mixed layer and the lower layer consists of both the wind mixing component involving the frictional velocity at the ocean surface a kraus turner parameterization and the convectively induced entrainment no dissipation of convective turbulence the latter is necessary in producing the chimney the results are an extension to the earlier model results hakkinen 1987 which did not include the convective component of entrainment

deep crustal structure of the precambrian basement beneath northern lake michigan midcontinent north america a deep seismic reflection profile in northern lake michigan midcontinent north america provides a cross section of the crust across the 1850 ma penokean orogen in which an early proterozoic island arc complex was deformed between two converging archean continental masses the island arc crust is about 40 km thick and has a few kilometres of intensely reflective rocks near its base above which it is variably reflective to transparent the archean terranes have thicker crust as much as 50 km the lower 20 25 km of which is strongly reflective abrupt offsets of moho near terrane boundaries may have been preserved since accretion during the penokean orogeny

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

centrifuge modelling of folding in high grade rocks during rifting centrifuge modelling is used to simulate the progressive development of structures in a simplified crustal profile during rifting this paper focuses on folding of ductile layers representing the middle to lower crust displacement along a pre existing cut in the layer representing mantle lithosphere creates a broad shear zone in overlying ductile layers and an asymmetric rift in upper layers the footwall of the mantle lithosphere layer and overlying shear zone are rotated to sub horizontal during rise of the basal ductile layer representing asthenosphere as an isostatic response to thinning in the extended model synthetic and antithetic faults develop in models with thicker ductile crustal layers although upper semi brittle layers representing upper crust constitute a simple fault bounded or sag basin the underlying ductile layers are complexly folded open upright folds developed over the crest of the footwall mantle lithosphere layer are tightened and increased in amplitude as they progressively rotate to a recumbent attitude refolding occurs during rise of the basal ductile material representing asthenosphere and from boudinage of overlying competent layers this study suggests that regional scale folds in some high grade terrains previously interpreted as evolving in a convergent margin tectonic setting may instead be produced as a result of flow of the ductile crust during rifting

a coupled one dimensional sea ice ocean model a prognostic one dimensional mixed layer pycnocline model describing the vertical structure of the upper ocean is coupled to a thermodynamic sea ice model the coupled prognostic model is compared with the more usual sea ice model overlying a fixed mixed layer with constant oceanic heat flux and is then applied to investigate the effect of temporal and regional variations of the entrained oceanic heat flux on the sea ice cover in both polar regions in the southern ocean a standard simulation is compared with two perturbation experiments which both describe the occurrence of a polynya in the arctic ocean the effect of possible soviet river diversions is investigated finally the response of the coupled model to paleoclimatic forcing and boundary conditions is presented

a new mechanism for decreasing north atlantic deep water production rates during the pleistocene geological data suggest that north atlantic deep water nadw production rates decreased during the pleistocene we apply a coupled ice mixed layer model for vertical overturn in the marginal ice zone hakkinen 1987a to propose an explanation for such a decrease the model predicts that prevailing wind direction is important in preconditioning of the water column along the marginal ice zone miz because it affects rates of upwelling and mixing of surface and subsurface waters processes that result in deep water formation at present prevailing wind directions during the winter in the greenland sea are northeasterly because ice has a larger drag coefficient than water ekman transport is greater under the ice than in open water this configuration causes divergence along the miz mixing of surface and subsurface waters and vertical overturn during the glacial maximum however the prevailing winds along the miz polar front were predominantly westerly modeling studies suggest that ice age changes in the prevailing winds would have significantly increased the depth of the mixed layer and decreased entrainment equal to mixing rates in agreement with geological data mixing changes due to westerly winds are greater than mixing changes due to a lower salinity layer under the present wind regime changes due to winds are so large that further changes in boundary conditions low salinity surface layer result in only minor additional reductions in vertical mixing still the maximum reduction occurs when the two processes are considered together a situation that may be applicable to the younger dryas event during the last deglaciation when a meltwater lid may have developed in the subpolar north atlantic our study suggests that wind induced changes in mixed layer depth along the miz represent a viable mechanism for decreasing past nadw production rates

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

crustal structure of the midcontinent rift system results from glimpce deep seismic reflection profiles interpretation of great lakes international multidisciplinary program on crustal evolution glimpce seismic reflection profiles indicates that the midcontinent keweenawan 1100 ma rift system of volcanic rocks and postvolcanic and interbedded sedimentary rocks extends to depths as great as 32 km about 10 5 s reflection time along profiles crossing western central and eastern lake superior and the northern end of lake michigan this area may overlie the greatest thickness of intracratonic rift deposits on earth times to moho reflections vary along strike from 11 5 to 14 s about 37 46 km depth in the west to 17 s about 55 km in the center and 13 to 15 s about 42 49 km in the eastern end of lake superior the prerift crust however was thinned 25 30 km beneath the central rift compared with its flanks providing evidence for crustal extension by factors of about 3 4 the midcontinent rift system differs from phanerozoic rifts in having total crustal thicknesses equal to or greater than the surrounding presumably unextended regions

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

proterozoic anorogenic magmatism and its orogenic connections there is a major belt of proterozoic so called anorogenic igneous rocks that extends from the baltic shield through southern greenland and eastern canada to southern usa all rocks are situated within a zone of 1 9 1 6 ga arc accretionary orogens that consist of juvenile mantle derived material especially accretionary prism sediments that provided the fertile source for abundant granitic crustal melts there were two main periods of formation of the anorogenic rocks 1 76 1 55 ga rocks are mainly rapakivi granites and rhyolites that formed about 60 200 ma after the last compressional events in the svecofennian ketilidian and penokean orogens the time lag being caused by the slow thermal relaxation of thickened lower crust that had commonly been depleted by the extraction of earlier granitic crustal melts 1 45 1 4 ga anorogenic rocks are mainly rhyolites and peraluminous granites in central southern usa and anorthosites in eastern canada that formed in the continental margin of the grenvillian ocean the subduction of which led to the grenville orogen these anorogenic igneous rocks formed in extensional regimes during the formation and breakup of a 1 5 ga supercontinent

the evolution of scaling laws in the sea ice floe size distribution the sub gridscale floe size and thickness distribution fstd is an emerging climate variable playing a leading order role in the coupling between sea ice the ocean and the atmosphere the fstd however is difficult to measure given the vast range of horizontal scales of individual floes leading to the common use of power law scaling to describe it the evolution of a coupled mixed layer fstd model of a typical marginal ice zone is explicitly simulated here to develop a deeper understanding of how processes active at the floe scale may or may not lead to scaling laws in the floe size distribution the time evolution of mean quantities obtained from the fstd sea ice concentration mean thickness volume is complex even in simple scenarios suggesting that these quantities which affect climate feedbacks should be carefully calculated in climate models the emergence of fstds with multiple separate power law regimes as seen in observations is found to be due to the combination of multiple scale selective processes limitations in assuming a power law fstd are carefully analyzed applying methods used in observations to fstd model output two important sources of error are identified that may lead to model biases one when observing an insufficiently small range of floe sizes and one from the fact that floe scale processes often do not produce power law behavior these two sources of error may easily lead to biases in mean quantities derived from the fstd of greater than 100 and therefore biases in modeled sea ice evolution

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

proterozoic extensional tectonics of the central fennoscandian shield results from the baltic and bothnian echoes from the lithosphere experiment a three dimensional crustal structure of the bothnian sea is deduced from the near vertical reflection data of the crosscutting lines 1 6 7 and c of the babel baltic and bothnian echoes from the lithosphere experiment for this study the common depth point stacked sections have been migrated with a velocity 6 5 km s and displayed as stacked envelope sections to show their enhanced large scale crustal structures the seismic sections show listric shear zones which dip toward se and flatten at major detachment zones 35 40 km and 48 km in depth the detachment zones are associated with changes in p wave velocity from 6 8 km s to 7 0 km s and from 7 4 km s to 8 2 km s and coincide with the middle and lower crustal boundary and the moho boundary respectively the geometry and crosscutting relationship of the detachment zone are well developed on the e w directed line 7 from where the moho detachment zone can be interpreted as to be younger different shear zone systems operated in the parallel n s directed lines 1 and 6 and therefore it is not easy to correlate crustal structures between lines 1 and 6 in the thinned southern part of study area the lower crust has a highly reflective lamellar structure and a strongly reflective subhorizontal moho increased reflectivity of the lower crust is attributed to stretching and mafic underplating of the crust associated with the subjotnian diabase dykes and rapakivi magmatism in the upper crust the rapakivi granite and diabase association is characterized by a rather transparent area within which strong reflections mimic graben and horst structures the distribution of the bimodal association seems to be controlled by the low angle listric shear zones the spatial distribution of outcropping lithologies combined with deeply penetrating shear zones suggests a simple shear extensional deformation the first stage of deformation which concentrated in the upper and middle crust records the collapse of the overthickened svecofennian crust the second stage involving the entire crust may record either the continuation of the postcollisional collapse or a separate anorogenic extensional period the seismic structures predict major post svecofennian deformation faulting and tilting in the central fennoscandian shield the extensional tectonics operating in the fennoscandian shield in the proterozoic is comparable to modern extensional tectonics

new u pb ages from the wiborg rapakivi area constraints on the temporal evolution of the rapakivi granite anorthosite diabase dyke association of southeastern finland abstract new u pb data on zircons monazites and baddeleyite suggest that the wiborg rapakivi batholith and associated mafic rocks in southeastern finland were emplaced mainly between 1650 and 1625 ma the earliest anorogenic magmatism related to the intrusion of the rapakivi granites was the emplacement of some diabase dykes about 1665 ma ago while the youngest porphyries intruded the rapakivi granites at 1615 ma the process involved three peaks of diabase activity at 1665 1645 and 1635 ma and two major granite events at 1640 5 and 1630 5 ma the former of which comprises also the intrusion of minor gabbroic anorthosite bodies on the whole the result was the emplacement of at least 105 km3 of rock material over a period of 50 ma within southern finland the rapakivi magmatism continued until 1540 ma by emplacement of the west finnish intrusions which combined are as extensive as the wiborg area globally the proterozoic rapakivi event probably represents the largest pulse of intracratonic magmatism which occurred during geological history and may be a consequence of rapid growth of continental masses in the early proterozoic

a coupled ice ocean model for the greenland iceland and norwegian seas abstract simulations from a coupled ice ocean model that highlight the importance of synoptic forcing on sea ice dynamics are described the ocean model is a non hydrostatic primitive equation model coupled to a dynamic thermodynamic sea ice model the ice modelling sensitivity study presented here is part of an ongoing research programme to define the role played by sea ice in the energy balance of the greenland sea the different categories of sea ice found in the subpolar regions are simulated through the use of equations for thin ice thick ice and the marginal ice zone a basin scale numerical model of the greenland iceland and norwegian seas has a horizontal resolution of 20 km and a vertical grid spacing of 50 m this resolution is adequate for resolving the mesoscale topographic structures known to control the circulation in this region the spin up reproduces the main features of the circulation including the cyclonic gyres in the norwegian and greenland basins and iceland plateau topographic steering of the flow is evident the baroclinic rossby radius of deformation is between 5 and 10 km so that the model is not eddy resolving the coupled ice ocean model was run for a period of two weeks the influence of horizontal resolution of the atmospheric model was tested by comparing simulations using six hourly wind fields from the ecmwf with those generated using six hourly fields from a hirlam with horizontal resolutions of 1 and 0 18 respectively the simulations show reasonable agreement with satellite ice compactness data and data of ice transports across sections at 79 n 75 n and denmark strait

heterogeneous stretching simple shear and basin development abstract the models of basin development which involve either homogeneous stretching of the whole lithosphere or displacement on a lithospheric scale shear zone are but two end members of a range of possible extensional models the homogeneous extension model thins the lower lithosphere beneath the thinned upper crust and superimposes a thermal subsidence basin on the earlier fault bounded basin the shear zone model offsets the zone of lower lithospheric stretching and thermal subsidence it is more likely that the zones of upper and lower lithospheric stretching will be heterogeneous and patchy but will often overlap in plan view this will produce localised uplift and subsequent thermal subsidence within the faulted basin and may explain many of the anomalies between the various stretching estimates made using different structural stratigraphic and geophysical techniques the model which combines heterogeneous lithospheric stretching and associated simple shear may explain 1 variations in dip of the major detachment zones in the large basins 2 variations in types of strain on or beneath the detachment zones 3 regional uplift of part of a basin to erode the earlier fault blocks and 4 the development of volcanism in basins with only low values of upper crustal extension the zone of stretched lower crust and lithospheric mantle may lie beneath the centre of the zone of upper crustal stretching or to one side it may be linked to the upper crustal zone by faults which dip consistently in one direction or by extensional systems which change their dominant dip direction with depth pulling out the mid crust as one or more wedges possible examples are discussed from the basin and range province and northwest european continental shelf

crustal structure of the northern baltic sea palaeorift abstract integrated analysis of the gravity data measured in the northern baltic sea in 1997 and the near vertical babel data shot in 1989 provides important constraints for the subjotnian 1600 1500 rift related evolution of the svecofennian orogenic crust the subjotnian is characterised by voluminous bimodal rapakivi granite magmatism that is manifested as large granite batholiths associated with ovoid crustal thickness minima and large bouguer gravity minima 20 to 40 mgal in the seismic sections the batholiths are transparent whereas the lower crust beneath is highly reflective and bears signs of intra and underplating the bouguer anomaly map together with the seismic results imply that the crust of the southern gulf of bothnia is mostly composed of subjotnian intrusions with a large rheologically stronger mafic svecofennian block in the middle the largest batholiths aland bothnian and rodon are mostly covered by water but even the minor plutons at stromsbro reposaari and siipyy are interpreted to be small satellites to larger marine batholiths a 3 4 km deep jotnian sandstone basin the strommingsbodan basin was developed on top of the bothnian rapakivi granite we suggest that it was initiated by cauldron subsidence in the postjotnian the granites and sandstones were intruded by diabase sills the contacts of which are imaged by high amplitude reflections the rapakivi granites are separated by a large transparent and dense body in the lower to middle crust it is interpreted as an older svecofennian mafic intrusion the northern baltic sea has many characteristic features of paleorifts topographic low now under water thin crust with large crustal thickness gradients and the products of voluminous bimodal magmatism the gulf of bothnia aborted rift is a part of a honeycomb like wide rift area that extends from lake ladoga to the caledonides 800 km 800 km it has seeds of many localised narrow rifts 100 km 100 km not connected at the lower crustal level

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

a coupled dynamic thermodynamic model of an ice ocean system in the marginal ice zone wind driven ice ocean processes in the marginal ice zone are studied when thermodynamics is included into a coupled ice ocean model the ocean is considered to be a two layer ocean where a fresh and cold freezing mixed layer is lying above the stagnant warm and saline lower layer the essential part of the thermodynamics is the entrainment between the mixed layer and the lower layer which is parameterized using the frictional velocity at the ocean surface a kraus turner type parameterization it is shown that upwelling at the ice edge which is generated by the difference in the air ice and air water surface stresses will give rise to a strong entrainment by bringing the pycnocline nearer to the surface furthermore the entrainment is negligible outside the areas affected by the ice edge upwelling when cooling at the top is included the heat and salt exchange are further enhanced in the upwelling areas because the entrainment depends inversely on the density difference between the layers new ice formation occurs in the area which is not affected by ice edge upwelling that is no entrainment of the warm lower layer fluid so the mixed layer temperature stays at the freezing point the ice growth will give rise to a large salt flux to the upper layer but the subsequent salinity change is not as large as that due to the entrainment of the lower layer fluid the mixed layer in the areas affected by the ice edge upwelling is only slightly less saline than the lower layer it is suggested that these high salinity mixed layer regions with a scale of a few rossby radii of deformation 10 km will overturn due to cooling possibly as a plume and may this contribute to the formation of deep water

tectonic styles of early proterozoic ore deposition in the fennoscandian shield the early proterozoic metallogenic events of the fennoscandian shield are connected with two major stages of crustal evolution the karelian continental development 2 5 2 1 ga and the svecofennian orogeny 2 0 1 75 ga the karelian continental development was dominated by deposition of terrestrial sediments and tholeiitic volcanism on archaean crust a tensional regime probably associated with a mantle plume and partial melting of the upper mantle culminated in the generation of an ene wsw trending rift zone which was intruded by tholeiitic magma 2 44 ga ago some 15 layered igneous bodies hosting cr pge ni cu and fe v ti deposits are known in the kemi suhanko and koillismaa belts these mafic ultramafic bodies originally formed either several lopoliths or one single layered igneous complex with a minimum length of 260 km nw se and nnw sse trending dolerite dykes intruded the rifted and attenuated archaean crust 2 25 2 0 ga ago preceding ocean opening the coeval ne sw trending dolerite dykes in the northern part of the shield are interpreted as manifestation of an aulacogen the failed arm of a triple armed rift system oceanic crust is preserved in 1 96 ga old ophiolite complexes the cu co zn ni and cr contents of the outokumpu type deposits derive from hydrothermal leaching of oceanic crust and were discharged together with chemogenic cherts and carbonates from ascending hydrothermal fluids on the seafloor above fissure vents in the northern part of the shield rifting and probably ocean opening resulted in the formation of the layered tholeiitic sedimentary complex of the petsenga ni cu district intruded by mineralized mafic ultramafic magma the age of this event is poorly constrained the basin was subsequently closed and continental collision of the kola suture was accompanied by southward obduction of the lapland granulite belt on the archaean crust during the svecofennian orogeny the divergent continental margin was converted into an active margin early subduction of oceanic crust produced oceanic island arcs intruded by 1 93 1 9 ga old tonalites and trondhjemites subsequent subduction of oceanic crust generated 1 9 1 87 ga old continental island arcs associated with massive sulphide cu zn pb ag au deposits the island arcs were accreted to the archaean crust in a concave belt containing the ore districts of kiruna skellefte vihanti pyhasalmi aijala orijarvi and bergslagen bimodal volcanic suites with felsic calc alkaline and mafic tholeiitic volcanites are attributed to rifting of the overriding plate above subduction zones ni cu deposits in synorogenic 1 9 1 87 ga old mafic ultramafic intrusions occur in linear fault controlled belts at the margin of the archaean crust the generation of ni bearing tholeiitic to komatiitic magma is explained by partial melting of the mantle wedge above the early subduction zone magma differentiation and magma upwelling along transcurrent faults on the continental margin sulphur was probably assimilated from the supracrustal wall rocks 1 89 1 87 ga old early svecofennian granitoids are associated with porphyry type cu mo au deposits widespread late svecofennian s type 1 84 1 75 ga granitic magmatism generated minor mo u w sn and ree deposits emphasis is laid on the importance of geotectonic models as a basis for further exploration in the fennoscandian shield

from collision to collapse phases of lithospheric evolution as monitored by seismic records abstract deep seismic reflection profiles in europe and elsewhere cover a range of different tectonic units specifically in western and central europe they cross structures relating to the alpine variscan and caledonian orogens with considerable crustal shortening delamination and interfingering the variscan mountain belts in france and germany show collapsed structures from various collision events between 300 and 350 ma ago still further north in middle england and the southwest baltic sea traces of the caledonian collision around 400 ma and associated collapse structures are visible here the terrain east avalonia cadomia docked to the colliding continents of baltica and laurentia in a complex pattern with closing oceans and compressional boundaries which can still be seen in today s seismic sections in britain and the sw baltic sea all the processes of crustal shortening interfingering and delamination were certainly active during the compressional stages of these earlier orogens and have left their marks which are still recognizable in today s seismic image of the crust crustal roots and high elevations have disappeared in the extensional collapse phase thermal events have intruded underplated or otherwise modified the stretched lower crust in the variscan internides massif granite production started and the lower crust assumed an especially strong and thick sheared laminated structure with a plane moho the various tectonic stages are illuminated by a gross analysis of reflectivity patterns we postulate that the fate of these patterns from their origin to their death is imbedded in thermally and rheologically varying creep processes which always accompany the brittle and ductile deformation in the earth s crust

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

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

the seasonal variation of the mixed layer and the pycnocline under polar sea ice a model of the seasonal variation of the mixed layer and the pycnocline under polar sea ice is compared with observations during the arctic ice dynamics joint experiment aidjex 1975 1976 the model is of the kraus turner type extended to include mixed layer and pycnocline the forcing mechanisms are the mechanical mixing due to ice keel stirring and the surface buoyancy flux due to melting and freezing of sea ice the model is designed to be used for climate studies in conjunction with atmospheric and oceanic general circulation models and with sea ice models

implications for proterozoic plate margin evolution from geophysical analysis and crustal scale modeling within the western gawler craton australia abstract complexly deformed late archean to mesoproterozoic rocks of the western gawler craton are poorly exposed thus reducing the ability to extract meaningful geological structural and tectonic information this study focuses on shear zone analysis via a constrained integration of qualitative aeromagnetic and bouguer gravity data interpretation with quantitative two dimensional forward modeling this integration provides a means to interpret the crustal architecture as well as constrain a sequence of events which aid the interpretation of a tectonic history for the western gawler craton results indicate a polyphase shearing history dominated by a crustal scale array of predominantly west dipping shear zones the initiation of east west trending sz1 structures was likely coincident with ca 1750 1720 ma crustal extension high heat flow and deposition on thinned areas of archean crust with the development of a dense possibly underplated lower crustal component sz1 r2 dextral transpressional movements reactivated sz1 during the ca 1720 1670 ma kimban orogeny which is interpreted to record collision between the gawler craton and the north australian craton during this event basins marginal to and in the interior of the archean crustal block of the western gawler craton were inverted and the dense lower crust was vertically offset causing some of the long wavelength 30 50 km bouguer gravity anomalies presently observed shear zone activity is constrained until ca 1680 ma during which time subduction rollback established a new arc position at the southern margin of the archean crust with magmatism persisting until ca 1670 ma sz3 structures overprint and offset the interpreted axis of the kimban orogeny within the archean crust these structures record a major phase of crustal scale sinistral strike slip movement associated with north south shortening between ca 1630 and 1540 ma prior to their reactivation at ca 1450 ma this sz3 r4 reactivation is interpreted to cause southwest directed crustal transport which was focused as west side up movements internal to the archean continent whereas dextral strike slip was focused at a major rheological boundary between thick archean crust to the south and thick paleoproterozoic basins to the north

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

middle proterozoic early palaeozoic evolution of central baltoscandian intracratonic basins evidence for asthenospheric diapirs abstract the three intracratonic sedimentary basins located in central baltoscandinavia namely the bothnian gulf basin the bothnian sea basin and the baltic basin developed in response to middle proterozoic and late proterozoic tectonic events separated in time by about 800 ma only the baltic basin was subsequently affected by caledonian orogenesis and mesozoic rifting crustal extension was minor or did not take place during the proterozoic basin evolution phases however according to the moho topography crustal thinning did take place this was probably a result of subcrustal magmatism on a craton wide scale the ages of granitoids which intruded during the middle proterozoic basin formation generally decrease from east to west this fact combined with the evidence provided by mantle derived flood basalt magmatism points to a moving asthenospheric diapir as the cause for basin development asthenospheric upwelling was probably also responsible for the second late proterozoic basin evolution phase as evidenced by the lack of crustal thinning and extension and the occurrence of tholeiitic intrusions in addition a late proterozoic thermally induced palaeo high located at about the position of the intracratonic basins is compatible with indications from glaciations as the ages of late proterozoic intracratonic basins also decrease from east to west across the craton the location of asthenospheric diapirism during this time interval was also moving for the fennoscandian lithosphere the presence of fundamental lithospheric weakness zones e g terrane boundaries might be an explanation for the formation of two generations of basins originating from asthenospheric upwelling at about the same location in the fennoscandian shield the spacing and size of the proterozoic intracratonic basins suggest that the asthenospheric diapirism was not deep seated therefore sublithospheric convective processes might be the cause for the asthenospheric upwellings such processes are related to rayleigh taylor instabilities in the sublithospheric mantle emplacement of an asthenospheric diapir causes a thermal bulge at the surface of the lithosphere modelling results demonstrate that erosion of the surficial high succeeded by cooling of the lithosphere can explain the accumulation of early palaeozoic sediments in the bothnian sea basin taking into account post ordovician vertical and lateral erosion of the basin fill

space time variation of mixed layer properties heat and salt fluxes and ice melt in the newfoundland marginal ice zone oceanographic data collected in the newfoundland marginal ice zone in march 1992 are analyzed to study the variation of mixed layer properties across the ice zone the mixed layer depth decreases from 80 m in the interior of the pack ice to 25 m at the eastern ice edge over a distance of 120 km in the open ocean without ice cover the depth is considerably greater 150 200 m the temperature under the pack ice is near freezing a sharp increase in both the temperature and salinity occurs across the ice edge the variation of the mixed layer properties is simulated by integrating a one dimensional bulk mixed layer model coupled to sea ice from november 1991 to march 1992 the initial ocean conditions are obtained from an objective analysis of archived data of november 1991 the forcings are 6 hourly meteorological data from the european centre for medium range weather forecasts ice concentration and thickness are treated as prescribed functions of time extracted from daily ice maps since ice in the area is not locally formed the model simulation is in good agreement with the observations from the model results a detailed analysis of the roles of wind stress ice melt surface cooling and shortwave radiation in the water mass transformation of the upper ocean is carried out there are large differences in the melt rate and water ice heat flux between the times when ice enters the shelf and after the water has been cooled by the ice to near freezing 15 20 cm d 1 and 500 700 wm 2 and 0 2 cm a 1 and 20 50 wm 2 respectively a large portion of the heat in the water column is expended for ice melting from january to march calculations that use surface data such as surface wind and sea surface temperature without considering ice melt can underestimate the net ocean to air heat flux especially at the eastern ice edge in march

a two dimensional thermodynamic model for sea ice advance and retreat in the newfoundland marginal ice zone a time dependent two dimensional ice ocean coupled thermodynamic model is developed to study the ice edge advance and retreat development of the oceanic mixed layer and oceanic response to ice movement ice melt and heat balance in the newfoundland marginal ice zone the model domain is a vertical section of the ocean along the direction of the ice velocity initially the ocean is free of ice and has a deep mixed layer formed by winter surface cooling an ice sheet then moves into the domain from the upstream boundary at constant velocity it melts from the bottom and a shallow mixed layer beneath the ice is developed heat and buoyancy fluxes at the air sea and ice water interfaces and at the bottom of the mixed layer determine the melt rate and change of the mixed layer properties the ice edge reaches a maximum distance and starts to retreat when the ice is being melted by the warm water faster than it is being advected into the area the oceanic properties change with the ice movement during the advancing phase of the ice movement the heat loss to the ice bottom from ice melting exceeds the heat gain by surface heating and entrainment causing the mixed layer temperature to drop but since the buoyancy created by the melting is not sufficient to overcome the effect of wind mixing the mixed layer deepens rapidly during the retreating phase the mixed layer becomes shallower and warmer because of the increasing surface heating and buoyance production by ice melting the most important factors controlling the melt rate and the excursion distance are the air temperature and the ambient water temperature higher wind speeds increase the mixed layer depth but do not have a strong effect on the melt rate

rock pulverization at high strain rate near the san andreas fault rocks near the san andreas fault are pervasively crushed at distances of up to 400 m from its core laboratory experiments and calculations suggest that the rocks were pulverized at high strain rates 150 s 1 associated with a supershear rupture a rupture propagating at a velocity equal to greater than that of seismic shear waves

comment on a coupled dynamic thermodynamic model of an ice ocean system in the marginal ice zone by sirpa hakkinen the recent article by hiikkinen 1987 creatively addresses a number of important dynamic and thermodynamic processes in the marginal ice zone miz we believe that her study will stimulate further research on the coupled ice ocean system for now we would like to offer the following discussion of the need to augment the entrainment equation to include surface buoyancy flux for general application to the miz

continental growth by accretion a tectonostratigraphic terrane analysis of the evolution of the western and central baltic shield 2 50 to 1 75 ga tectonostratigraphic terrane analysis of the western and central baltic shield defines a framework for continental growth through the period 2 50 1 75 ga eight discrete terranes can be defined in this part of the svecokarelian orogen the older cratonic kuhmo and iisalmi terranes the hybrid lapland and savo province allochthonous terranes the juvenile island arc like skellefte savonlinna and south finland central sweden terranes the back arc or arc like outokumpu nappe and the ophiolitic oceanic jormua nappe stitching events can also be recognized these include the ca 1 88 1 87 ga svionian magmatic arc the ca 2 44 go koillismaa intrusions the kalevian flysch basin and the bothnian basin constraining the age of these stitching events permits the construction of an accretion history in which the svecokarelian orogeny can be resolved as a number of accretion events deformation episodes related to accretion or magmato tectonic episodes these events defined here include the pre 2 44 go pohjolan accretion of the component parts of the lapland hybrid terrane the ca 1 95 ga kyllikian accretion of the karelian collage the ca 1 90 ga karelian orogeny marked by the development of the savo thrust belt in response to the accretion of the skellefte savonlinna terrane to the karelian collage and the ca 1 88 1 87 ga svionian magmatic arc the whole svecokarelian collage had assembled by ca 1 85 ga an amalgamation succeeded and stitched by a diverse collection of igneous and thermal events through the period 1 85 1 75 ga

the specific case of the mid proterozoic rapakivi granites and associated suite within the context of the columbia supercontinent abstract rapakivi granites and the associated suite anorthosite mangerite and charnockite that developed during the mid proterozoic escape the global rules that control usual granitic magma formation about 40 points that characterise the proterozoic magmatism confirm the originality of the magmatism rapakivi granites developed without a global orogenic context within the supercontinent columbia the intrusions extend from the east european shield to the western us through fennoscandia greenland and labrador other occurrences in the amazonian shield australia or south china are less well documented they show no definite trend in age or chemistry that would explain large scale mantle plume effects a mantle upwelling of material is contradictory with the re assembly of a supercontinent because it occurred before this magmatic episode 1 9 1 8 ga at 1 7 ga the supercontinent columbia amalgamated with the collision of the yapavai and mazatzal provinces a model is suggested that takes into account the supercontinent re assembly defining a downwelling flow in the mantle that anchors the continent above it in contrast to a material flow heat is still delivered to the base of the continental lithosphere and is focused toward the juvenile suture zone the base of the crust must reach 1200 1300 c before producing anorthosite magmas under such a high temperature gradient lasting over a long time magmas are transferred toward the upper crust giving the thin 5 km and square shape 100 km that the intrusions presently have heat delivery is essentially conductive leading to long time spans for intrusions the presence of the supercontinent immobile over a descending cell poloidal mode of convection developed a tangential force toroidal mode of convection that partly split the continent through strike slip deformation due to plate rotation it developed progressively between 1 57 and 1 3 ga starting from fennoscandia and then passing to amazonia western us and labrador in a clockwise sense the associated rotation induced sinistral shear manifested by small scale shear zones and the orientation of late magmatic facies topaz bearing granites in each province the proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons the present moho still shows remnants of this process having bumpy undulations that may reach 22 km in amplitude over a distance of 200 km

modeling the thermodynamics of a sea ice thickness distribution 2 sea ice ocean interactions in this paper we examine the coupling of an ice thickness distribution model with an ocean mixed layer model the annual cycle of the modeled mixed layer temperature salinity and depth are compared to observations from the drifting ice stations of the arctic ice dynamics joint experiment aidjex the role of the ice thickness distribution in determining the ice ocean coupling is examined we find that the ice thickness distribution is important for the exchange of heat salt and fresh water between the ice and the ocean especially with regard to the effects of thin first year ice several different parameterizations of the ice ocean turbulent heat exchange are compared the results indicate that the storage of heat in the ocean mixed layer is an important physical process for determining the annual average ice thickness double diffusion and the stabilizing effects of meltwater also impact the exchange of heat between the ocean and ice cover questions remain as to the relative importance of these processes and their accurate parameterization in ice ocean coupled models