heat flow in the baltic shield results of the lithospheric geothermal modelling abstract the results of heat flow measurements performed within the baltic shield and in its closest neighbourhood are reviewed the regional distribution of the heat flow density is presented as a heat flow map the characteristic heat flow density amounts to 35 40 mw m 2 and is notably smaller than heat flow in the surrounding areas the radiogenic heat production within the crust contributes to the observed surface heat flow by about 15 20 mw m 2 this permits to estimate the moho heat flow in the most stable parts of the shield to about 20 mw m 2 and over 25 30 mw m 2 in the surrounding platform to the south and east relatively low moho temperatures of 350 500 c characterize the crust mantle boundary of the shield increasing to 600 700 c beneath the shield margins the thermal thickness of the lithosphere of the shield can attain more than 200 km

thermomechanical lithospheric structure of the central fennoscandian shield abstract the deep seismic sounding dss profiles baltic including its southern continuation the sovetsk kohtla jarve skj profile sveka the northern part of babel polar fennia and pechenga kovdor kostomuksha were used in studying the present day thermomechanical structure of the central fennoscandian shield these profiles are located in different tectonic units which represent different stages in precambrian crustal and lithospheric growth first present day geotherms were constructed for several points along the dss profiles successively strength envelopes were calculated using the obtained geotherms and rheological flow laws variations in strain rate were also considered in the computations of the strength envelopes the integrated crustal and lithospheric strengths the thicknesses of the mechanically strong crust msc and mechanically strong lithosphere msl and the rheological thickness of the lithosphere were derived from these strength envelopes the obtained mechanical structures for different regions were analysed and compared with other geophysical data e g seismicity depth and isotherm depth distributions the rheological results show lateral variations in the lithospheric strength reflecting the geometry of the lithosphere and following roughly the same trend as the geochronological development of the fennoscandian shield the mechanical structure shows distinct decoupling of the weak lower crust and the strong upper mantle particularly with a wet rheology this decoupling interrupts the transmission of the differential stress from the brittle upper crust to the ductile lower crust and through it to the mantle lithosphere the weak lower crustal layer is also detected with a dry rheology in the svecofennian area whereas in the archaean side it is not distinct the assumed frictional transition temperature of 350 c varies between the depths of 25 and 44 km with an average value of 35 km this is in good agreement with the observed focal depth limit of 31 km consequently it seems that the velocity weakening velocity strengthening explains best the real lower boundary of seismicity

palaeoclimate and structure the most important factors controlling subsurface temperatures in crystalline rocks a case history from outokumpu eastern finland summary the present work contributes to the study of heat transfer mechanisms in crystalline bedrock we present evidence from a thoroughly investigated case history in outokumpu eastern finland in the fennoscandian or baltic shield which shows that the subsurface temperature field is controlled by the thermal conductivity structure and downward diffusion of palaeoclimatic ground temperature variations the subsurface temperature profiles from three continuously cored boreholes 790 1 100 m deep were used for a detailed 2 d modelling of structural hydrogeological and palaeoclimatic effects on the subsurface temperature field the boreholes are situated in a subdued topography and they intersect early precambrian folded lithologies such as mica gneisses black schists skarn rocks quartzites and serpentinite talc rocks finitedifference techniques were used in the numerical solution of the heat and masstransport equations the 2 d models of thermal conductivity and hydraulic permeability that were compiled were based on an extensive set of data relating to geological structure in situ hydraulic permeability groundwater composition and thermal conductivity of drill core samples down to about 1 km depth it was found that the thermal effect of topographically driven groundwater convection is very small with peclet numbers typically of the order of 10 4 10 5 the effect of anisotropy of thermal conductivity was found to be one order of magnitude smaller than the effects of heatflow refraction in the inclined rock layers which produce horizontal components of heat flow density ranging from 15 to 5 mw m basal heat flow 35 mw m since the advective heat transfer could be neglected we tried to find a palaeoclimatic groundtemperature history that would explain the measured data first an inversion algorithm based on heat conduction in laterally homogeneous media was used for the reconstruction of palaeoclimatic ground temperature history but it yielded spurious results because of the strong 2 d conductivity effects the only means to reconstruct the past ground temperature changes was forward modelling using a conductive transient 2 d model with time dependent surface temperature variations the surface temperatures at different times between 100000 yr bp and the present and the basal heat flow density were varied in order to reach a reasonable fit between the modelled and measured borehole temperatures to reach this goal the ground temperature history must include the main climatic events during the last 100000 yr such as the last glacial epoch and the little ice age from about 1300 ad to 1700 ad followed by warmer temperatures in more recent times our results suggests that subsurface temperatures in conditions similar to those of the outokumpu case can yield a wealth of palaeoclimatic information when an appropriate approach for modelling heat transfer in the bedrock is chosen

radiogenic heat production in the variscan crust new determinations and distribution models in corsica northwestern mediterranean abstract new heat production values of variscan volcanic rocks cropping out in the southern part of the corsican batholith were determined in the laboratory by analysing uranium thorium and potassium concentrations with gamma ray spectrometry they vary from a minimum of 0 2 for basalts to a maximum of 4 2 w m 3 for calc alkaline granites the th u ratio in variscan granitoids is slightly higher than normal generally our measurements yield values higher than those previously determined these data together with those available for the variscan massifs of the northwestern mediterranean were used for constructing models of distribution of radiogenic heat production of the crust the lithologic composition was deduced from available p wave velocity data by taking into account the pressure and temperature effects depending on the rate of decrease with depth of the radiogenic heat the crustal contribution in terms of heat flux ranges from 26 to 39 mw m 2 and on average is 33 mw m 2 the mean heat production of the crust is 1 0 w m 3 both average crustal heat source and crustal heat contribution are quite similar to those estimated with the heat production seismic velocity relationship

variation of radiogenic heat production rate in 2 8 1 8 ga old rocks in the central fennoscandian shield abstract we present results of a geochemical survey based on 1150 outcrop samples for heat production studies in a e w oriented band 120 km 500 km located approximately between 62 and 63 n in finland the central fennoscandian shield the results provide representative averages of heat production rate in the studied tectonic units but also demonstrate the heterogeneity and spatial variation in radiogenic heat production rate of the precambrian lithosphere the study area covers formations from the archaean granite greenstone terrain in the east via a palaeoproterozoic mobile belt to a major granitoid complex surrounded by schist belts in central and western finland in this study we review the heat production rates as calculated from the u th and k total analyses and densities of the samples and the relations of heat production rate values with major tectonic setting lithological types major composition and petrophysical properties of rocks the data is further correlated with heat flow values measured in boreholes and applied in a lithospheric heat production model in the central part of the shield generally heat production rate increases from archaean to proterozoic rocks in e w direction but this trend is relatively weak and often overrun by lithological variations the arithmetic means are 1 1 1 3 w m 3 in the archaean domain 1 3 0 6 w m 3 in the hoytiainen autochthonous proterozoic domain 1 2 0 5 w m 3 in the suvasvesi domain characterized by allochthonous proterozoic cover 1 0 0 6 w m 3 in the proterozoic raahe ladoga mobile belt 1 3 1 1 w m 3 in the proterozoic rantasalmi haukivuori domain 1 6 0 8 w m 3 in the central finland granitoid complex and 1 4 0 8 w m 3 in the bothnian schist belt the standard deviations of the mean values are considerable and reflect the heterogeneity of heat production all distributions overlap heat production rate shows spatial variations which range in scale from kilometres to tens of kilometres well defined systematic variations of heat production rate values with either sio2 content density or p wave velocity could not be found although the correlations are statistically significant at 1 risk level the correlations are weak and scattered and sometimes opposite in sign in the major rock groups plutonic metavolcanic and metasedimentary heat flow density measured in boreholes increases with increasing heat production rate but the present data do not support a strict linear dependence between the variables and the regression line parameters vary depending on the applied heat production values drill core analyses versus hole site averages from the present study this indicates that the heat production rate is spatially heterogeneously distributed in the scale of the boreholes and their formations a lithospheric heat production model was constructed using data from the present study for the upper crust literature data for lower crustal xenoliths in the study area and a xenolith derived heat flow density value for the mantle the total crustal contribution to the surface heat flow density 37 mw m 2 from radiogenic heat sources is calculated as 26 mw m 2 and the contributions from the lower middle and upper crust are 4 11 and 11 mw m 2 respectively

geochemistry and origin of saline groundwaters in the fennoscandian shield chemical and isotopic analyses of water from drill holes and mines throughout the fennoscandian shield show that distinct layers of groundwater are present an upper layer of fresh groundwater is underlain by several sharply differentiated saline layers which may differ in salinity relative abundance of solutes and o h sr and s isotope signature saline groundwater can be classified into four major groups based on geochemistry and presumed origin brackish and saline waters from 50 200 m depth in coastal areas around the baltic sea exhibit distinct marine chemical and isotopic fingerprints modified by reactions with host rocks these waters represent relict holocene seawater inland three types of saline groundwater are observed an uppermost layer of brackish and saline water from 300 900 m depth saline water and brines from 1000 2000 m depth and superdeep brines which have been observed to a depth of at least 11 km in the drill hole on the kola peninsula u s s r electrical and seismic studies in shield areas suggest that such brines are commonly present at even greater depths the salinity of all inland groundwaters is attributed predominantly to water rock interaction the main solutes are cl ca na and mg in varying proportions depending on the host rock lithology the abundance of dissolved gases increases with depth but varies from site to site the main gas components are n2 ch4 up to 87 vol and locally h2 the 13c value for methane is highly variable 25 to 46 and it is suggested that hydrothermal or metamorphic gases trapped within the surrounding rocks are the most obvious source of ch4 the uppermost saline water has meteoric oxygen hydrogen isotopic compositions whereas values from deeper water plot above the meteoric water line indicating considerably longer mean residence time and effective low temperature equilibration with host rocks geochemical and isotopic results from some localities demonstrate that the upper saline water cannot have been formed through simple mixing between fresh water and deep brines but rather is of independent origin the source of water itself has not been satisfactorily verified although superdeep brines at least may contain a significant proportion of relict precambrian hydrothermal or metamorphic fluids

heat flow and thermal structure of the lithosphere across the baltic shield and northern tornquist zone abstract terrestrial heat flow data from archean and proterozoic provinces of the baltic shield its southwestern margin and the tectonic transition to the younger tectonic units of central europe northern segment of the european geotraverse are compiled and examined for local and regional variations a 2 d numerical lithospheric temperature and heat flux model is presented covering 1800 km of the deep seismic fennolora profile across the baltic shield and 500 km of eugeno s northern eugemi seismic lines across the northern tornquist zone and the danish and north german basins thermal lithosphere defined as the outer layer with relatively low temperatures t c in which heat transfer is dominantly by conduction is found to vary markedly in thickness from 85 110 km beneath basins and tornquist zone to about 150 km in central shield areas and 200 250 km in areas of low heat flow along the northern shield profile these results are in very good agreement with seismological information from surface wave dispersion analysis and the refraction wide angle seismic interpretation of fennolora data the combined thermal and seismologic evidence indicates that the uppermost mantle region stratified in seismic velocity is restricted to the upper relatively cold thermal layer which is interpreted to constitute shield lithosphere local and to some extent regional variations in surface heat flow are found to be closely related to variations in near surface radiogenic heat production for the southern baltic shield a heat flow heat production relationship of q 0 32 8 7 6 h 0 is derived regional increase in heat flow from northern and central shield provinces to the shield margin and the tornquist zone is interpreted to be related partly to differences in upper crustal heat generation partly and in southernmost areas mainly to an increased heat flux from the upper mantle upper mantle heat flux is estimated at 20 25 mw m 2 along the northern and central shield transect increasing to 30 40 mw m 2 beneath the shield margin basins and tornquist zone lithospheric thickness variations in this region of northern europe are found to be closely associated with variations in heat flux from the uppermost mantle and perhaps to a large extent controlled by geodynamically determined lateral differences in upper mantle heat flux earthquake focal depth data and information on crustal strength show baltic shield seismicity to be limited generally to the upper 20 25 km brittle part of the crust at temperatures less than 300 400 c maximum source depth of magnetic anomalies at temperatures near 600 c is in the deepest part of the crust near the crust mantle boundary in the southern region about 25 km depth along the tornquist zone and about 35 40 km in southern shield areas and in the uppermost mantle 60 70 km depth in the northern shield areas some petrologic implications are discussed the present deep crust is supposed to be at lower temperature pressure conditions than at early stages of formation however also the present deepest parts of the shield crust 35 40 km are within the stability field of eclogites the general absence of significant gravity anomalies in areas of marked crustal thickness variations indicate high density eclogites to be present in significant amounts in the deep shield crust

the configuration sensitivity and rapid retreat of the late weichselian icelandic ice sheet abstract the fragmentary glacial geological record across the icelandic continental shelf has hampered reconstruction of the volume extent and chronology of the late weichselian ice sheet particularly in key offshore zones marine geophysical data collected over the last two decades reveal that the ice sheet likely attained a continental shelf break position in all sectors during the last glacial maximum though its precise timing and configuration remains largely unknown within this context we review the available empirical evidence and use a well constrained three dimensional thermomechanical model to investigate the drivers of an extensive late weichselian icelandic ice sheet its sensitivity to environmental forcing and phases of deglaciation our reconstruction attains the continental shelf break across all sectors with a total ice volume of 5 96 105 km3 with high precipitation rates being critical to forcing extensive ice sheet flow offshore due to its location astride an active mantle plume a relatively fast and dynamic ice sheet with a low aspect ratio is maintained our results reveal that once initial ice sheet retreat was triggered through climate warming at 21 8 ka bp marine deglaciation was rapid and accomplished in all sectors within c 5 ka at a mean rate of 71 gt of mass loss per year this rate of ice wastage is comparable to contemporary rates observed for the west antarctic ice sheet the ice sheet subsequently stabilised on shallow pinning points across the near shelf for two millennia but abrupt atmospheric warming during the bolling interstadial forced a second dramatic collapse of the ice sheet onshore with a net wastage of 221 gt a 1 over 750 years analogous to contemporary greenland rates of mass loss geothermal conditions impart a significant control on the ice sheet s transient response particularly during phases of rapid retreat insights from this study suggests that large sectors of contemporary ice sheets overlying geothermally active regions such as siple coast antarctica and ne greenland have the potential to experience rapid phases of mass loss and deglaciation once initial retreat is initiated

highly varying radiogenic heat production in finland fennoscandian shield abstract radiogenic heat production in finland has been previously studied using airborne gamma ray surveys and glacial till measurements alike for the first time this paper presents a detailed survey on the spatial variation in radiogenic heat production determined using outcrop samples obtained from all important lithologies of the country the dataset of 6465 samples represents mostly mesoarchean about 2 7 ga paleoproterozoic ca 2 2 1 8 ga and mesoproterozoic ca 1 6 1 3 ga rocks nearly all data are from precambrian fennoscandian shield area but heat production appears to be highly variable and above global archean and proterozoic averages spot readings show an arithmetic average of 1 34 1 19 wm 3 and a range from 0 02 to 19 4 wm 3 the interpolated areal average of the whole area is 1 42 1 41 wm 3 the high standard deviation of data is related to the geochemical characteristics of uranium u thorium th and potassium k resulting in a skewed distribution of heat production mesoproterozoic anorogenic rapakivi granites and late paleoproterozoic svecofennian granitoids show the highest heat production values in the range of 3 5 wm 3 the results show no distinct dependencies of heat production with geological age metamorphic grade nor seismic p wave velocity but an increasing trend of heat production with sio2 content and decreasing trends of heat production with fe2o3 content and with rock density are evident surface heat flow 44 borehole data values correlates weakly with heat production r 0 35 the general heterogeneity of heat production calls for supporting information from other geophysical methods for better understanding of the thermal state of the lithosphere in finland and beyond

crustal structure of the baltic shield based on off fennolora refraction data abstract since the early 1960s deep seismic sounding experiments have been carried out on the baltic shield in this study we will mainly concentrate on the results obtained from two international profiles sveka and baltic carried out in finland in 1981 and 1982 results from these profiles are shown and discussed and compared with those obtained from the fennolora and from the other recent refraction profiles of the baltic shield in fennoscandia according to the results from sveka and baltic and average crustal velocity is 6 6 6 7 km s which is rather high several distinct reflection boundaries have been found within the crust in the lower part of the crust a high velocity layer with a p wave velocity of 7 0 7 5 km s has been found in some cases in addition the results indicate that the crustal structure has a clear block like character different blocks being separated from each other by deep fractures the crustal thickness in the baltic shield is about 45 km on average whereas around the ladoga bothnian bay zone in central finland it is about 10 km thicker than this thus there is a large scale depression in the moho boundary in the central part of the baltic shield

87sr86sr and ree variations along the easter microplate boundaries south pacific application of multivariate statistical analyses to ridge segmentation abstract we report 87 sr 86 sr ree sc v cr and co analyses on 48 basaltic glasses dredged at 47 different localities along the spreading boundaries of the easter microplate emp the latitudinal 87 sr 86 sr variation along the east and west rifts closely parallels that of the pb isotope variation previously published and confirms the strong abnormality of the mantle underlying this part of the east pacific rise epr the data also further support the binary mantle mixing model previously proposed we evaluate the potential of multivariate statistical analyses in studying this and previously published geochemical data obtained on these very same 48 basaltic glasses the data base includes major and trace elements ranging from compatible e g cr to highly incompatible e g la and the pb and sr isotope ratios multivariate classification of these geochemical variables in q mode sample space allows to clearly distinguish major element grouping from those of moderate to highly incompatible elements and the isotope ratios reflecting mantle source heterogeneities present in the region the power of this multivariate geochemical classification is best represented in two dimensional space by combining principal components analysis pca with cluster analysis ca illustrated in the form of a dendrogram the method is further illustrated by classifying elements of the lanthanide series at different statistical level of affinities using the variance covariance matrix and correlation matrix in r mode we further demonstrate a close relationship between segmentation revealed by the multiple variation of the four isotope ratios and the actual tectonic segmentation both discriminant analysis and pca further indicate a close relationship between the geochemical anomaly located on the east rift of the emp and sala y gomez and easter island to a lesser extent the tuamotu chain whereas there is no close affinity between the emp and the more distant juan fernandez san felix or society islands in the isotopic space considered this provides further support that the geochemical anomaly located on the east rift of the emp may be related to the influence of a plume located either beneath the east rift or in the vicinity of sala y gomez or easter island in the last two models the plume would be laterally deflected at shallow depth towards the epr emp spreading complex an analysis of statistical generalized distances against geographical distances rules out that such influence is caused by radial dispersion of this plume in the upper mantle the flow between the plume and the east rift of the emp must be more confined or channelled the possibility that the plume would be located directly beneath the east rift cannot be ruled out but appears less likely

a heat production model of a shield area and its implications for the heat flow heat production relationship the pikwitonei and sachigo subprovinces of central manitoba provide a cross sectional view of the superior province crust to constrain thermal models of the crust we measured heat production and thermal conductivity in rocks from this terrain using standard gamma ray spectrometry and divided bar techniques these data when used in a two dimensional model of shield crust show that estimates of surface heat flow and surface heat production yield the well known linear relationship between surface heat production and surface heat flow observed for heat flow provinces this demonstrates that determinations of heat production with depth based on inversion of the linear heat production heat flow relationship are non unique

audiomagnetotelluric measurements across the lake ladoga bothnian bay zone in central finland summary a group of 64 stations among 140 amt tneasurenlent sites in central finland forms a profile about 220 km long the profile starts from the svecokarelidic orogeiiic belt 1 800 1900 ma and traverses the karelidic schist belt to the presvecokarelidic basement 2600 2800 ma the data obtained are analysed and are presented as apparent resistivity profiles pseudosections and residual profiles ai discrete frequcncies between 8 and 3700 hz the distribution of the measured apparent resistivities in the research area shows that there are observable differences between different geological units a i d interpretation is carried out at every station after which a 2 d model is constructed for the whole profile and its amt response is calculated numerically with the finite element technique

anomalously low heat flow density in eastern karelia baltic shield a possible palaeoclimatic signature abstract we report new heat flow density hfd values in seven drill holes in the kamennye lakes area in eastern karelia russia approximately at latitude 63 15 n longitude 36 10 e the investigated holes are 250 750 m deep and they intersect archaean ultrabasic serpentinites and talc carbonate rocks measured gradients range from 0 8 to 3 7 mk m 1 and the apparent hfd values from 2 4 to 11 6 mw m 2 the holes are not technically disturbed by fluid flow or any drilling effects average heat production of the rocks as analysed in the core samples of the deepest measured hole is 0 25 w m 3 but the low heat production is not a critical factor in producing the low hfd values this is due to refraction of heat as shown with 2 d conductive simulations of heat transfer in a low heat production formation surrounded by higher heat production hydrogeological disturbances can be ruled out by the presence of saline groundwater in the sections deeper than 150 400 m and low topographic variation in the area as well as peclet number estimates which suggest negligible convective heat transfer in the bedrock all the temperature profiles are curved indicating recent palaeoclimatic disturbances inversion studies with singular value decomposition techniques yielded a climatic warming of about 1 0 1 5 k which started 150 200 years ago and was preceded by a cool period which lasted about 100 years nevertheless recent climatic changes cannot explain the very low apparent hfd values but long period effects of the weichselian glaciation are sufficient to decrease the hfd values to the levels measured these effects were investigated with forward simulations and suggest that present temperature gradients in the range of 1 4 mk m 1 in the uppermost 1 km can be created by a very cold ground temperature 10 to 15 c during the glaciation time 60 11 ka ago

terrestrial heat flow and groundwater circulation in the bedrock in the central baltic shield abstract unpublished heat flow measurements from 17 drillholes in finland central baltic shield are presented the holes range from 270 to 1080 m in depth sharp vertical changes in the heat flow density and in the temperature gradient indicating groundwater flow disturbances were detected in eight holes the variations in heat flow were typically of the order of 5 10 mw m 2 in many cases the groundwater flow concept was supported by geological logs and other borehole measurements which indicated the existence of fracture zones at the same depths as the flow the flow zones were usually encountered at depths of less than 500 m but flows between fracture zones in the holes were detected even deeper at as much as 930 m according to recent groundwater studies in the same holes the water flowing in the fracture zones seems to be fresh or only slightly saline in composition this can be taken as an indication of relatively rapidly circulating groundwater in contrast to the more saline and more stagnant groundwater encountered deeper in the holes the results indicate that disturbances in heat flow induced by groundwater flow may be much more common in areas of crystalline bedrock than has previously been realized the apparent heat flow densities mean 35 0 3 0 mw m 2 agree well with values published previously from the central baltic shield but the results from southern and western finland indicate regionally above average heat flow values 45 mw m 2

magnetotelluric and audiomagnetotelluric measurements in finland abstract joint finnish hungarian mt magnetotelluric and amt audiomagnetotelluric measurements were carried out in finland in the framework of the international elas project the conditions for mt measurements are favorable at these latitudes five mt and 150 amt stations gave information on the electrical conductivity distribution in the area amt results guided the choice of mt sites with minimal near surface distortion effects and helped the interpretation of the mt soundings the mt measurements indicate the presence of large conductivity anomalies and can be best interpreted as lateral induction effects of near surface dyke structures this result is confirmed by a certain correspondence between the directions of the maximum impedances and of the tectonic zones of the area any information about the upper mantle would require the use of sq harmonics because of the crustal conductivity anomalies detected by the mt measurements

hydrological disturbances of the conductive heat flow in stable continental crust abstract geophysical data generally support the presence of free aqueous fluids in the crust to at least the depth of the 700 750 c isotherm where granulite facies conditions are reached which are presumed to have no free water the fluid phase distribution and transport of fluid in rocks strongly depend on the rheological conditions in the lithologically heterogeneous crust changes in rheological regime can produce quite complex sandwich like fluid transport layering including domains of upward migration very impermeable layers or thin reservoirs with nearly horizontal fluid transport in the present study two two dimensional thermal models of the stable continental crust with different spatial configurations of fluid systems have been calculated the results show that such systems redistribute heat in the crust and give rise to zones of concentrated heat energy storage and transfer the pattern of the disturbances to the conductive heat flow depends on the arrangement of the fluid systems which is a function of the crustal structure temperature and regional stress field and shows significant lateral and vertical variations even at very low velocities of the fluid percolation a few millimetres per year the disturbances to the conductive heat flow may reach 30 60

lithospheric thermal structure in the baltic shield summary steady state heat conduction modelling was carried out to calculate the crustal temperature field and thermal lithosphere thickness in the baltic shield the radiogenic sources at the surface were fixed depending on the age of the crust below the uppermost 10 kilometres a relationship between heat generation and p wave velocity was applied and seismic structure was used to define individual crustal blocks of specific thermal parameters small scale surface heat flow density anomalies are interpreted as lateral variations of heat generation within the upper part of the crust whereas the large anomaly in the southern part of the shield is attributed to an anomalously high mantle heat flow density the results are shown on maps outlining mantle heat flow density moho temperature and thermal lithosphere thickness a relationship between subcrustal temperature and moho depth was found except for southern sweden the lithospheric thickness is found to exceed 200 km in the bothnian gulf northern central finland and part of the kola peninsula where temperatures at the moho are less than 500 c the temperature gradient at the top of the mantle has on the average a value of 7 5 mk m and the mantle heat flow density varies from 19 to 25mwm 2 towards the south the lithospheric thickness decreases until it attains a value lower than 100km under southern sweden in this latter area the moho temperature and mantle heat flow density are remarkably high 700 900 c and 30 45 mw m 2 respectively as well as the temperature gradient which amounts to 10 mk m

geothermal modelling of the lithosphere in the central baltic shield and its southern slope lithospheric temperature and heat flow density hfd were studied in the central baltic fennoscandian shield and its subsurface continuation to the south along a transect trending from eastern finland to southern estonia the transect represents an example of a low hfd 30 mw m 2 archaean craton on a thick 150 190 km lithosphere surrounded by early and middle proterozoic mobile belts on a thinner 110 150 km lithosphere with slightly elevated hfd 35 55 mw m 2 numerical 2 d conductive models were constructed in which peridotite solidus temperatures were assigned to those depths which correspond to the seismically determined lithosphere asthenosphere boundary this technique was found to reduce the effect of uncertainties in heat production and thermal conductivity values on the simulation results upper crustal heat production values for the finnish terrain were taken from published geochemical analyses of outcropping rocks for the estonian terrain new heat production values were measured from core samples representing nineteen deep boreholes middle and lower crustal lithologies were estimated with the aid of the deep seismic vpvs data and corresponding heat production values were adapted from global xenolith averages and from data for granulites cropping out in other precambrian areas the results of the modelling suggest that the lithosphere and moho depth variations are only weakly reflected in the measured surface heat flow data which are mainly controlled by heat sources in the upper crust the simulated heat flow densities at 50 km depth approximately at the moho are relatively low and range from 12 mw m 2 at the archaean northeastern end to 19 mw m 2 on the proterozoic southwestern end of the transect simulated temperatures at 50 km depth increase from northeast to southwest ranging from 450 550 c in eastern finland to about 650 c in estonia sensitivity of the simulations to parameter changes was studied by varying the heat production and thermal conductivity values the extreme values for the moho temperature estimates thus obtained may be about 50 k lower or 100 k higher than the values above the corresponding sensitivity of the moho hfd is about 6 mw m 2 and of the surface hfd 5 20 mw m 2 respectively

the precambrian continental crust its structure composition and evolution as revealed by deep drilling in the u s s r abstract petrologic geochemical studies of the kola superdeep and other deep borehole sections drilled in the precambrian basement of the east european platform are discussed proterozoic metasediments and metavolcanic rocks that constitute the pechenga palaeorift and the underlying archaean granite gneiss basement were detected by the kola borehole in the 0 6842 m and 6842 12064 m intervals respectively zones of density decrease by hydrofracture resulting from progressive metamorphism under closed system conditions were identified in the upper crust at a 4500 9000 m depth sharp textural variations of rocks an increase in porosity and the presence of mineralizing fluids are specific features of these zones which may have contributed to tectonic deformation and metamorphically produced ore formation deep boreholes drilled into the more eroded archaean basement of the tatar dome provided information that was used for constructing a petrologic geochemical model of the lower crust to a depth of 25 30 km adacitic trondhjemitic composition for the primeval continental crust was calculated by extrapolation of geochemical trends established for the lower crust we propose that the lower crust formed as a result of an initial melting of the earth s chondrite shell and only existed in pre archaean times we also present data on temperatures and distribution of radiogenic elements in the kola and some other borehole sections a heat production model is proposed for the archean crust according to the model the crustal contribution to the heat flow is 25 mw m 2 or 52 of the total heat flow and that of the mantle is 22 24 mw m 2 or 48 of the total flow

lithospheric thermal regimes in europe abstract the heat flow pattern of europe derived from more than 8000 observations is dominated by a general northeast to southwest increase in geothermal activity which is the consequence of the tectonic evolution of the whole continent this paper reviews the history of the geothermal mapping of europe and describes the recently compiled set of geothermal maps of europe surface heat flow is correlated with the age of the last tectonothermal event with the near surface heat production and with the crustal thickness deep temperature distributions within the crust lithosphere are calculated for 1 d 2 d and simple 3 d models characteristic temperature depth curves are given for some of the major tectonic provinces and the results of various techniques are compared the value of the crustal heat contribution is assessed and a regional moho heat flow pattern is proposed the mohorovicic discontinuity is clearly not an isothermal surface neither is the heat flow from below the crust constant

heat flow heat production and thermo tectonic setting in mainland uk new heat flow data for the united kingdom together with additional heat flow and heat production determinations for caledonian age granites have led to a revision of the uk heat flow map and a re examination of the relationship between heat flow q 0 and heat production a 0 for granites and basement rocks previously recognized broad belts of above average heat flow are now resolved into separate zones which reflect to a greater extent the geological structure and tectonic history of the uk the zones of highest heat flow are spatially associated with voluminous high heat production granitoid batholiths in sw england northern england and the eastern highlands of scotland a single linear correlation between q 0 and a 0 is no longer tenable and an analysis in terms of broad heat flow provinces each with a characteristic upper crustal heat production distribution and deep heat flow contribution is also considered to be an oversimplification on the q 0 a 0 plot the data form four separate clusters three corresponding to the granite batholiths in sw england northern england and the eastern highlands of scotland and the fourth to the basement rocks of central england and wales an explanation of the q 0 a 0 data is proposed in terms of the crustal structure and thermo tectonic setting of each area in the case of the granite batholiths the data reflect the contrasting depth extent and radioelement depth functions of the intrusions these parameters in turn are related to the magmatic evolution and emplacement history of each batholith and the nature of the crust into which they were emplaced

radiogenic heat production in the continental crust abstract the thermal structure and evolution of continents depend strongly on the amount and distribution of radioactive heat sources in the crust determining the contribution of crustal rocks beneath a superficial layer is a major challenge because heat production depends weakly on major element composition and physical properties such as seismic wavespeed and density enriched granitic intrusives that lie at the current erosion level have a large impact on the surface heat flux but little influence on temperatures in the deep crust many lower crustal rocks that are poor in heat producing elements are restites from ancient orogenic events implying that enrichment of the upper crust was achieved at the expense of deeper crustal levels for the same total heat production concentrating heat sources in an upper layer acts to reduce temperatures in the lower crust thereby allowing stabilization of the crust the present day structure of the crust is a consequence of orogeny and should not be adopted for thermal models of the orogenic event itself this review summarizes information extracted from large data sets on heat flow and heat production and provides estimates of crustal stratification and heat production in several geological provinces analysis of global and regional data sets reveals the absence of a positive correlation between surface heat flow and crustal thickness showing that the average crustal heat production is not constant differences of heat flow between geological provinces are due in large part to changes of crustal structure and bulk composition collating values of the bulk crustal heat production in a few age intervals reveals a clear trend of decrease with increasing age this trend can be accounted for by radioactive decay indicating that thermal conditions at the time of crustal stabilization have not changed significantly for the average crustal thickness of 40 km moho temperatures are near solidus values at the time of stabilization suggesting an intrinsic thermal control on crustal thickness and heat production distribution crustal thickening by more than about 10 km above this mean value induces changes of gravitational potential energy that exceed the strength of the lithosphere for several provinces where strong constraints on heat production are available it is shown that prior to intracrustal fractionation only modest amounts of thickening were needed to generate the conditions of ultra high temperature metamorphism the tell tale signature of crustal heat production is anatectic and metamorphic events that lag the cessation of orogenic activity by several tens of million years crustal heat production is decreasing with time because it is due to the radioactive decay of uranium thorium and potassium its rundown is responsible for the secular cooling of lithospheric roots at a typical rate of about 100 k gy 1 implying complex thermal interactions with a convecting mantle that is not cooling at the same rate

thsr isotopic relationships in morb abstract although morb are well known to exhibit a limited range in isotopic ratios such as 87sr 86sr a compilation of existing 230th 232th data which reflect present th u in the source shows a large dispersion this is in contrast to the admittedly limited data available for oceanic islands for which there is a broad correlation between 87sr 86sr and 230th 232th the distribution of data for morb is approximately bimodal with relatively depleted samples plotting from within to well above the band defined by the oceanic island correlation 87sr 86sr 0 70230 0 70270 and 230th 232th 1 1 2 3 and relatively enriched morb plotting from within to below this band 87sr 86sr 0 70270 70320 and 230th 232th 0 8 1 4 the data are consistent with the generally accepted idea that e morb are generated by the interaction of depleted mantle with more enriched material having higher th u and 87sr 86sr however the large range in source th u implied by the thorium isotopic compositions of both enriched and depleted morb requires complex melting and or other recent source modification processes much of this variability may be caused by multiple small degree melt extraction at the spreading ridge a process which would affect all incompatible trace element concentrations and ratios

introduction background and implications of the linear heat flow heat production relationship the discovery in the late 1960s of the linear heat flow heat production relationship was a major advance in the understanding of the thermal structure of continents however although heat flow provinces have now been defined in all continents except antarctica the full implications of the relationship to deep thermal structure and the limitations of the relationship to applications in all terranes are not yet clear in addition despite major advances in the geochemical factors controlling the distribution of heat production in the crust a geochemical rationale for the relationship is lacking the relationship was originally interpreted in terms of one dimensional models but new two and three dimensional model studies suggest more complex interpretations in terms of lower crustal heat flow contributions to these problems in this special issue represent new observational geochemical and modeling studies of the relationship and a renewed attempt to understand the thermal and geochemical implications of the relationship

uzth isotopic systematics at 13 n east pacific ridge segment fresh basaltic glasses have been analyzed for uzth disequilibrium systematics as part of an extensive study on the east pacific rise epr at 12 45 n these samples are well described in terms of major and trace elements as well as in nd pb and sr isotopes our results show significant heterogeneities in the mantle source at a small scale and show heterogeneities at larger scales also when compared to other epr data u and th concentration and isotopic data rule out fractional crystallization as a main process and support a mixing model in agreement with the marble cake model developed by alle gre and turcotte and constrained by trace elements and nd sr and pb isotopes on the same samples by prinzhofer et al based on the high 230th 232th isotopic ratios on recent tholeiites especially the th u values inferred for their sources clearly show that the upper mantle th u has decreased with time

crustal heterogeneities and the thermal structure of the continental crust the observed linear relationship between surface heat flow q0 and the radiogenic heat production of near surface rocks a0 discovered almost two decades ago has been a primary tool used to infer the distribution of heat production in the continental crust recent evidence on the nature of compositional heterogeneities in the continental crust has led us to reexamine the q0 a0 relationship and the likely implications for the thermal structure of the lower continental crust numerical simulations of crustal thermal models indicate that some interpretations derived from this relation may be flawed in particular the length scale determined from the q0 a0 function normally thought to describe the depth distribution of crustal heat sources is significantly affected by the horizontal scale of crustal heterogeneity as a consequence most estimates of lower crustal heat flow and lower crustal temperatures obtained using heat flow data with the q0 a0 relationship are overestimates this overestimation is proportionally largest in areas of low mantle heat flux such as cratons where basal heat flux may be overestimated by more than a factor of two

submarine magma degassing and explosive magmatism at macdonald tamarii seamount macdonald seamount is an active volcanic centre located at 29 98 s 140 25 w in the south central pacific since its discovery1 in 1968 a number of expeditions have surveyed and dredged rocks from the seamount during a scripps institution of oceanography expedition in october 1987 the first direct observation of volcanic activity at macdonald was made when a small eruption occurred spewing forth scoriaceous basaltic rocks which briefly floated to the sea surface the rocks were accompanied by huge bubbles of volcanic gas that created large sea surface slicks h craig personal communication similar activity at macdonald was observed by a joint french german team in january 1989 using the submersible cyana2 here we look at the radio geochemical features of the lone rock recovered from the october 1987 eruption we present analyses of u series nuclides from the first observed eruption of macdonald also known as tamarii seamount with interpretations focusing on the timescales of mag matic events and the implications of uranium series data for magma degassing this highly gas charged magma lost all of its volatile 210po upon eruption based on this estimates of the fluxes of other volatile elements to sea water which may contribute significantly to their oceanic budget are calculated high 230th 232th and 87sr 86sr ratios common signatures of altered crust suggest that the explosive nature of the macdonald volcan ism results partly from the sealing of magma conduits by seawater circulation

heat flow map of northern and central parts of the fennoscandian shield based on geochemical surveys of heat producing elements abstract the heat flow heat production q a relationship is a useful tool in geothermal research and it has been widely used for delineating geothermal provinces and determining characteristic parameters of heat production in the continental crust in this study a simple but rarely used technique of utilizing the heat flow heat production relationship is discussed in central and northern parts of the fennoscandian shield extensive geochemical surveys have produced 1483 samples taken from glacial till with a sampling density of 1 sample 300 km 2 heat production values determined from u th and k concentrations in these samples were used to calculate a map of heat flow density a previously determined q a relationship q 15 8 10 8 a was applied the compiled map covers all finland northern sweden and northern norway about 35 of the exposed shield area in fennoscandia heat flow density and heat production increase with decreasing geological age and correlate with granitoid types the calculated heat flow density values on the map were controlled with 12 drill hole measurements not used in calculating the applied q a regression line nine of them are from previously unpublished data from the finnish part of the shield the agreement with drill hole measurements and the geochemical estimate is reasonable although not perfect in all cases the differences can be attributed to anomalous vertical variation in heat production reliability of the applied q a plot reliability of till geochemistry in bedrock studies convective groundwater disturbances or local structural effects the calculated heat flow map can be used as a data set supplementing drill hole measurements to determine representative values of heat flow density in areas with low numbers of drill hole measurements

terrestrial heat flow and radiogenic heat production in finland the central baltic shield abstract heat flow density and radiogenic heat production of the bedrock were studied in finland in part of the central baltic fennoscandian shield heat flow data were collected from 35 holes 270 1080 m deep the heat production values at the sites were determined from the drill core samples by gamma ray spectrometry the areal variation in heat production was studied with the aid of k u and th analyses of 1054 glacial till samples collected for the geochemical atlas of finland a heat production map constructed from this data set revealed a strong areal variation that can be attributed to the known lithological geochemical and tectonic features of the bedrock in general heat production seems to increase with decreasing geological age the presented data strongly suggest that the southern and western parts of finland are geothermally anomalous apparent heat flow density 38 68 mw m 2 surface heat production 2 0 w m 3 in contrast to the other parts of the country mw m 2 w m 3 the heat production heat flow density plots were constructed from apparent and palaeoclimatically corrected heat flow densities the heat production values were determined from both drill core samples and till samples the plot constructed from the till data and apparent heat flow densities which yielded a scale depth of 10 8 2 6 km and a reduced heat flow density of 15 8 4 9 mw m 2 was considered the most reliable the results are discussed in terms of crustal fracturing and deep groundwater flow if the crystalline bedrock is approached as a medium consisting of blocks internally conductive but bounded by fracture zones in which heat is transported by groundwater flow an alternative hypothesis can be constructed for interpreting the linear heat production heat flow density plots according to this hypothesis the scale depth and reduced heat flow densities refer only to the geothermal properties of the uppermost bedrock block in the present study this concept is supported by results from deep electromagnetic and seismic soundings

heat production in the continental crust part i data converted from seismic velocities and their attempted interpretation abstract when using the simple exponential distribution of radiogenic heat production a with depth z a z a o exp z d the statistical analysis of a large number of heat production profiles converted from the observed seismic velocity vs depth profiles and the experimental knowledge of the relationship between seismic velocity and heat production disclosed certain relations between the logarithmic decrement d value and heat flow tectonic age and crustal thickness the d valuein this respect characterizes the depth rate of the differentiation of the heat production in all tectonic units the d parameter decreases with depth as the next step the more complicated distribution law a z a o exp z d a was applied where a characterizes the quantity of removed heat sources by the processes accompanying the crustal evolution such as remelting metamorphosis leaching by underground waters etc all parameters a o a and d are constant within each particular tectonic province and parameter d does not depend on depth contrary to the former law the amount of removed heat sources a reflects the initial concentration of heat sources and is higher in young tectonic terranes in areas of high heat flow and in regions of thin crust

the vertical distribution of radiogenic heat production in the precambrian crust of norway and sweden geothermal implications the present geology of southern scandinavia offers the unique opportunity to sample deep and intermediate levels from the same crustal section for both heat flow and heat production in the central part of southern norway amphibolite facies terranes appear to lie on top of the same deeper crustal formations which crop out on their western and eastern margins an extensive data set on the geochemistry of all types of rocks in the region culled from the literature is used to derive values for radiogenic heat production for each kind of crustal facies using constraints from heat flow data in the same area allows a reliable model of the distribution of crustal heat production the average heat production of granulite facies terranes is 0 4 w m similar to values in other parts of the world for amphibolite facies rocks the value is 1 6 w m the present shield also includes heat producing element enriched granites formed in later events and the average heat production of presently exposed crust is 2 7 m using heat flow and radioactivity data from ten stations the reduced heat flow in the area is 22 2 mw m this corresponds to the heat flow at the top of 28 km of deep crustal facies implying that the mantle heat flow is probably as low as 10 mw m over the whole crustal thickness the average amount of radiogenic heat is 31 mw m

structural expression of oblique seafloor spreading in the macquarie island ophiolite southern ocean the macquarie island ophiolite is an uplifted block of oceanic crust formed at the australia pacific spreading center between 12 and 9 ma the sense of motion and geological processes across this plate boundary reflect an evolution from orthogonal spreading through progressively more oblique spreading to the present day transpressional regime the crust that makes up the island was formed during an interval of oblique spreading along east trending spreading segments punctuated by a series of northwest trending discontinuities the discontinuities are accommodation zones marked by oblique slip dextral normal faults localized dikes and lava flows and extensive hydrothermal alteration indicating that these zones were active near the spreading axis these features provide a window into the internal structure of oceanic crust generated by oblique spreading