the major ion composition of silurian seawater one hundred fluid inclusions in silurian marine halite were analyzed in order to determine the major ion composition of silurian seawater the samples analyzed were from three formations in the late silurian michigan basin the a 1 a 2 and b evaporites of the salina group and one formation in the early silurian canning basin australia the mallowa salt of the carribuddy group the results indicate that the major ion composition of silurian seawater was not the same as present day seawater the silurian ocean had lower concentrations of mg2 na and so42 and much higher concentrations of ca2 relative to the ocean s present day composition furthermore silurian seawater had ca2 in excess of so42 evaporation of silurian seawater of the composition determined in this study produces kcl type potash minerals that lack the mgso4 type late stage salts formed during the evaporation of present day seawater the relatively low na concentrations in silurian seawater support the hypothesis that oscillations in the major ion composition of the oceans are primarily controlled by changes in the flux of mid ocean ridge brine and riverine inputs and not global or basin scale seawater driven dolomitization the mg2 ca2 ratio of silurian seawater was 1 4 and the k ca2 ratio was 0 3 both of which differ from the present day counterparts of 5 and 1 respectively seawaters with mg2 ca2 2 e g modern seawater facilitate the precipitation of aragonite and high magnesian calcite therefore the early paleozoic calcite seas were likely due to the low mg2 ca2 ratio of seawater not the pco2 of the silurian atmosphere

internal deformation in clay models of extension by block faulting clay and sand have provided useful analogues for the failure by block faulting of continental crust under extension we performed experiments on clay models of such faulting and found that the total horizontal displacement on the major faults provided only 40 50 of the known extension similar experiments on sand showed that the displacement provided 70 80 of the known extension examination of the differential tilt in both models also underestimated the extension the hidden extension is due to internal deformation within the blocks assuming that the sand and clay deform exclusively by slip and rotation on four families of faults and that these faults have many scales we propose that the hidden extension occurs on faults below the visible scale we observed much of the deformation in sand because the grain size the lower limit for faulting was close to the minimum observable scale for the model on the other hand in clay two or three orders of faults lay below this scale giving rise to a hidden extension of 50 or more we cannot apply our results directly to the block faulting behavior of continental crust however our method of analysis suggests an approach that may have value deformation within the continental crust occurs at many scales some of these scales remain hidden when determining the amount of extension in such cases our postulated behavior for clay may present a useful analogue as it suggests that deformation within the blocks may play an important role for example estimates of extension found by summing the horizontal displacement on normal faults observed on multichannel seismic lines are lower than those given from analyses of the subsidence of many sedimentary basins and continental margins hidden deformation at a scale below that observable on the seismic line could account for this difference

the importance of small scale faulting in regional extension a recurring observation in many studies of extensional basins has been that the amount of extension visible on normal faults for example on seismic reflection profiles is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1 5 one mechanism suggested to account for this discrepancy is small scale faulting with offsets too small to be resolved seismically6 7 but earthquake studies8 10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area scholz and cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements here we present fault displacement data that directly conflict with scholz and cowie s conclusions and imply that up to 40 of the extension may be missed by summing fault offsets on basin profiles the fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time

age relations among oslo rift magmatic rocks implications for tectonic and magmatic modelling abstract this paper presents a compilation of 67 published and unpublished radiometric age determinations by the rb sr method on magmatic rocks in the continental oslo rift in southeast norway the data include most major rock types and structural units in the subaerial part of the rift the implications of these data with respect to the tectonomagmatic evolution of the rift and processes in the mantle and crust are discussed the oslo rift was active for about 60 million years the earliest recorded magmatic activity was the intrusion of basaltic and syenitic sills in the pre rift lower paleozoic sediments at about 300 ma extrusion of plateau lavas of basaltic and intermediate composition rhomb porphyries started about 295 ma and lasted to about 275 ma with largest magma volumes and highest extrusion rate from 295 to 285 ma in the vestfold graben segment gs one flow per 250 000 years and 0 30 km3 per 103 years the main faulting activity and graben formation occurred during this period the plateau volcanism was succeeded by a stage of bimodal central volcanoes most of which later underwent caldera collapses ring dykes and central intrusions in these calderas give ages in the range 268 266 ma in the vestfold and 266 243 ma in the akershus gs large composite intrusive complexes were emplaced in the periods 27 268 and 273 241 ma in the vestfold and akershus gs respectively a rate of propagation of the rift from ssw towards the nne has been estimated to 1 2 cm y 1 on the basis of 87 sr 86 sr initial ratios and type of magmatic products we estimate magmas to have been emplaced into the crust only during the first 20 25 ma of the magmatic period some of the magmatism appears to be initiated by movement along precambrian fault zones which were reactivated during the oslo rifting event

thermal consequences of lithosphere extension over continental margins the initial stretching phase summary we compute the thermal evolution of a lithosphere submitted to stretching during a finite duration of time in order to discuss the initial stretching phase of future continental margins the numerical method developed can handle 2 d laterally variable stretching as well as sedimentation it is shown that lateral conduction is more important than vertical conduction over most continental margins during their formation by stretching a simple way to evaluate the relative importance of lateral and vertical conduction effects at the axis of the zone of rifting just prior to oceanization is proposed a simple way to evaluate the amplitude of the thermal uplift on the edges of the zone of rifting at the end of the stretching phase is also presented for small width zones of rifting 70 100 km lateral cooling becomes so large as to prevent large scale melting and presumably prevent the transition to oceanization the effect of highsedimentation rates 100 500 m myr 1 is to increase the surface temperature of the lithosphere and consequently significantly decrease the surface heat flow

the origin and significance of groundwater seepage gypsum from bristol dry lake california usa gypsum and anhydrite fabrics observed in trenches and deep 500 m cores from bristol dry lake california usa exhibit a vertical alignment of crystals similar to the fabric seen in bottom nucleated brine pond gypsum however geochemical and sedimentological evidence indicate that the gypsum formed in bristol dry lake precipitated displacively within the sediment where groundwater saturated with respect to gypsum recharges around the playa margin groundwater seepage gypsum evidence for displacive growth of gypsum is i the geometry of the deposit ii stable isotopic data and the water chemistry of the brine and iii inclusions of matrix which follow twin planes and completely surround crystals as they grow the bulk of the gypsum precipitated in the playa occurs around the edges of the playa in the playamargin facies and completely rings the lake sulphate concentrations in the groundwater increase toward the gypsum zone in the playa margin basinward of this zone sulphate concentrations decrease sharply to trace element levels in the basin centre brine authigenic gypsum is rare in the centre of the playa stable 18o values measured for gypsum waters of crystallization gwc are similar to the values calculated for groundwater in the playa margin and alluvial fan sediments 6 0 whereas measured brine 18o values range from 0 5 to 3 7 0 deuterium values measured for groundwater are 70 0 gwc are 60 to 65 0 and brine values are 57 0 the geometry of the deposit and the chemical data suggest that the water precipitating the gypsum is more closely associated with the groundwater than the brine however some mixing between groundwater and brine is likely within 100 m of the surface the gypsum dehydrates to anhydrite although the same vertically aligned fabric is retained through the diagenetic process the similarity of displacive vertically aligned gypsum and anhydrite fabrics seen in bristol dry lake to subaqueously deposited gypsum in modern brine ponds indicates that the criteria used to define subaqueous fabrics must be better constrained

extracting mg isotope signatures of ancient seawater from marine halite a reconnaissance abstract the mg ca ratio of seawater fluctuated remarkably over the phanerozoic the processes that controlled the temporal variation in seawater mg ca ratio may also have caused the secular changes in mg isotope ratios of seawater therefore the mg isotope composition of seawater may be useful to understand the long term variation in seawater chemistry different sedimentary carbonates have been proposed as archives of ancient seawater mg isotope signatures but each type of carbonate has its limitations halite is a common mineral in evaporite deposits and their fluid inclusions may be used to infer the mg isotope composition of contemporary seawater in this study we developed a method to measure the isotope composition of mg in halite with an accuracy of better than 0 1 in 26mg the accuracy of the method was verified systematically using synthetic nacl solutions as well as holocene lacustrine halite samples from the dead sea the 26mg values of marine halite samples show a large variation ranging from 0 1 to 1 6 relative to the dsm3 international mg isotope standard the 26mg values of some ancient salt samples are lower than a published model curve for 26mg of seawater and such inconsistency could be explained by 1 isotopic evolution of brines due to mineral precipitation 2 entrapment of evaporite minerals into halite during halite precipitation or post depositional deformation 3 recrystallization and dissolution of pre existing mg minerals and 4 limitations of the model curve of seawater 26mg values collectively we suggest that mg isotopes in halite are a new geochemical proxy to study the secular variations in seawater chemistry syndepositional and post depositional processes of evaporite deposits our data highlight the importance of understanding the sedimentary background and textural details of evaporites for appropriate interpretations of mg isotope data from halite

a long geoclimatic record from the permian a 260 000 year continuous annual geoclimatic time series has been compiled from the varved permian castile and bell canyon formations of the delaware basin southeastern new mexico and southwestern texas the last 200 000 years of the record was obtained from calcite laminated anhydrite and anhydrite laminated halite which formed as a result of seasonal and annual deposition in the evaporite basin the time series is based on measured thickness of individual varves analyses of calcium sulfate calcium carbonate and organic matter were made on 50 year intervals in the evaporite part of the sequence time series of the major components were examined by using smoothed graphic plots variance spectra and moving correlation coefficients the longest oscillation recorded in the series that may be of climatic origin has a period of about 100 000 years the calcium sulfate record contains 9 11 distinct oscillations with an average period of about 20 000 years the strongest oscillation has a preferred spectral period of 2700 years and is recorded throughout the series as thickness changes in calcium sulfate as events controlling halite deposition and as episodes of basin freshening a broad spectral response near a period of about 200 years reflects changes in sulfate carbonate and halite thickness shorter periods do not have consistent spectra the time series contains no preferred periods that have not already been identified in previously described records the climatic changes appear to be the result of a deterministic response to orbital effects at the longer periods and stochastic processes at the shorter periods an unusually strong response near 2700 years is associated with episodic freshening of the basin this period has been well established in the holocene paleoclimatic record and its presence in the permian suggests a deterministic origin

sr nd and pb isotope geochemistry of the oslo rift igneous province southeast norway abstract sr nd and pb isotopic compositions are presented for a series of 290 to 250 ma old basaltic to granitic rocks from the oslo rift southeast norway a large group of basalts larvikites and rhomb porphyry lavas cluster within a restricted range of initial isotope ratios tsr 3 to 16 tnd 3 3 to 4 2 206 pb 204 pb i 18 9 to 19 3 207 pb 204 pb i 15 59 to 15 66 208 pb 204 pb i 38 6 to 39 1 the remainder of the rocks trend towards higher initial sr and lower initial nd and pb isotopic ratios data are tested against mantle crust mixing models the oslo rift magmatic rocks originated from a somewhat heterogeneous mildly depleted source located in the subcrustal lithosphere the isotopic character of this source has been inherited by the rocks showing uniform isotopic ratios which have thus not suffered significant crustal contamination the rest of the mafic to intermediate rocks and the syenites were contaminated in the intermediate to lower crust which increased slightly their sr but decreased their nd and pb isotopic ratios as much as 40 50 contamination is suggested for some of the samples strong increases in radiogenic sr seen primarily in the granites are attributed to contamination in the upper crust the mildly depleted mantle proposed as the source for the oslo rift mafic rocks has an isotopic character which can only be explained in terms of a multistage history which included periods of relatively high u pb and th pb ratios resulting from a relatively short depletion history in the subcontinental lithosphere and metasomatic enrichment of the mantle in selected elements

criteria for the recognition of salt pan evaporites layered evaporites can accumulate in 1 ephemeral saline pans 2 shallow perennial lagoons or lakes and 3 deep perennial basins criteria for recognizing evaporites deposited in these settings have yet to be explicitly formulated the characteristics of the ephemeral saline pan setting have been determined by examining eight holocene halite dominated pans salt pans and their deposits marine and non marine from the u s mexico egypt and bolivia these salt pans are typified by alternating periods of flooding resulting in a temporary brackish lake evaporative concentration when the lake becomes saline and desiccation which produces a dry pan fed only by groundwater the resulting deposits consist of alternating layers millimetres to decimetres of halite and mud the layers of halite are characterized by 1 vertical and horizontal cavities rounded crystal edges and horizontal truncation surfaces due to dissolution during flooding 2 vertical chevrons and cornets grown syntaxially on the bottom during the saline lake stage 3 halite cements overgrowths and euhedral cavity linings and disruption of layering into metre scale polygons produced during the desiccation stage the muddy interbeds are characterized by displacive growth of halite during the desiccation stage immediately below the surface of the pan the halite layers are matured by repeated episodes of dissolution and diagenetic crystal growth this results in porous crusts with patches of chevron and cornet crystals truncated by dissolution clear diagenetic halite cement and internal sediment these layers of mature halite closely resemble the patchy cloudy and clear textures of ancient halite deposits holocene salt pans are known to cover thousands of square kilometres and cap halite deposits hundreds of metres thick so they are realistic models for ancient evaporites in scale e g permian salado formation of new mexico texas which preserves many primary salt pan features

displacive halite hoppers from the dead sea some implications for ancient evaporite deposits abstract large hopper shaped halite cubes up to 10 cm on a side grow displacively within holocene sediments near the southern end of the dead sea under conditions of high temperatures ionic concentrations and rapid rates of crystallization two theories of formation are presented downward diffusion of ions versus upward diffusion and evaporative pumping both alternatives are considered possible and the correct choice probably depends upon the location of the sediments at the time of precipitation whether subaqueous or subaerial a choice indeterminate at present because of human disturbance to the sediments prior to collection displacively formed halite crystals similar in morphology to the dead sea cubes are common in evaporite bearing carbonate rocks of many regions such as the salina fm u silurian michigan basin the kirkham mudstone u triassic cumbria england and the u miocene gessoso solfifera fm of sicily these are believed to have formed under conditions analogous to those in the present dead sea these observations taken together with those of other authors may now be used for the facies interpretation of ancient halite deposits

physical changes in the lithosphere associated with thermal relaxation after rifting we define the terms modem rift and paleorift with respect to the thermal state of the lithosphere the former being rifted lithosphere not in thermal equilibrium and the latter being thermally stabilized rifted lithosphere a comparison of the physical characteristics of modern rifts with paleorifts shows that they share many of the same geomorphic structural and magmatic features but have important differences in their geophysical signatures using a simple model for the thermal relaxation of the lithosphere in the transition from modern rift to paleorift we show that differences in upper mantle seismic velocities and density surface heat flow thermal structure and curie point depth and magnetotelluric properties of the lithosphere between modern and paleorifts can be explained by lithospheric cooling cooling alone cannot explain the complex subsidence histories associated with and following rifting however nor the presence of high velocity crustal layers and sometimes thickened crust in paleorifts these features can be explained by consideration of lithospheric flexural strength and magmatic thickening of the crust associated with rifting we speculate that the anomalous upper mantle beneath many modern rifts may be associated with the process of magmatic thickening of the rifted crust

lithospheric stretching and north sea jurassic clastic sourcelands uniform lithospheric stretching cannot explain three important aspects of north sea basin evolution namely 1 the rapid growth of middle jurassic uplands from which were liberated major fluvial deltaic clastic wedges now forming the main hydrocarbon reservoirs 2 the occurrence of basin centre upwarp and basaltic volcanism and 3 widespread and significant 1 km subsidence outside the area of demonstrable crustal thinning i present here the suggestion that a combination of nonuniform stretching and lithospheric flexuring accounts for these fine tuning effects of the uniform stretching model with magnitudes close to those demanded by independent geological evidence

composition of brines in halite hosted fluid inclusions in the upper ordovician canning basin western australia new data on seawater chemistry analyses of primary and early diagenetic fluid inclusions in the halite from the late ordovician mallowa salt canning basin western australia indicate a ca rich composition and high concentration of parent brines in the basin which were close to sylvite and carnallite precipitation the salt bearing series in the sampled interval was overheated up to 62 c the recorded differences in gas compositions result from the input of several gas sources including dispersed organic matter in the salt series and hydrocarbon deposits in the underlying rocks the high concentration of the brines in fluid inclusions does not allow quantitative reconstruction of the chemical composition of late ordovician parent seawater using the information from early cambrian and late silurian basins as a proxy however the new data indicate that late ordovician seawater was undoubtedly ca rich and in comparison with modern seawater had a similar k content considerably lower mg content c 30 approximately three times the ca content and one third the so4 content

mass estimates of cumulates and residues after anatexis in the oslo graben the magmatic province of the permian oslo rift southeast norway is dominated by rocks of monzonitic to granitic composition most of the monzonitic rocks are believed to have formed by fractional crystallization from mantle derived magmas whereas melts produced by anatexis in the lower crust are important to the formation of the syenitic and granitic rocks these processes have left dense cumulates and dense residues after melting in the crust the surplus masses of cumulates and residues at depth in the crust can be estimated by using petrological and geological information on the rocks exposed today quantitative estimates for the oslo graben are comparable to corresponding data derived from geophysical crustal models the petrological geological estimates suggest excess masses of 7 10 1010 kg per linear meter assuming prismatic bodies along the rift axis in the southern and 5 7 1010 kg m in the northern part of the oslo region these estimates are dependent upon the volume proportion of trapped liquid in the cumulates and the ratio of cumulates to residues after anatectic melting in the crust the amount of trapped liquid appears to be low less than 25 the results agree well with estimates by wessel and husebye 1986 of excess mass in the crust based on gravimetric data which gave 5 7 1010 kg m this agreement testifies to the validity of making mass estimates on the basis of petrological and geological data the data bear on the composition and evolution of the upper lithosphere in the graben area some cumulates may reside in the subgraben mantle

halite saltern in the canning basin western australia a sedimentological analysis of drill core from the ordovician silurian mallowa salt the late ordovician early silurian mallowa salt of the carribuddy group canning basin north west australia is the largest halite deposit known in australia attaining thicknesses of 800 m or more within an area of approximately 200 000 km2 study of 675 m of drill core from bhp utah minerals brooke no 1 well in the willara sub basin indicates that the mallowa salt accumulated within a saltern dominantly subaqueous evaporite water body that was subject to recurrent freshening desiccation and exposure textures and bromine signatures imply a shallow water to ephemeral hypersaline environment typified by increasing salinity and shallowing into evaporitic mudflat conditions toward the top of halite mudstone cycles type 2 and the less common dolomite anhydrite halite mudstone cycles type 1 the borate mineral priceite occurs in the capping mudstones of some cycles reinforcing the idea of an increasing continental influence toward the top of mudstone capped halite cycles the rock salt in both type 1 and type 2 cycles typically comprises a mosaic of large randomly orientated interlocking halite crystals that formed during early diagenesis it only partially preserves a primary sedimentary fabric of vertically elongate crystals some with remnant aligned chevrons intraformational hiati halite karst tubes and solution pits attest to episodic dissolution stacked type 2 cycles dominate occasional major recharges of less saline perhaps marine waters in the same area produced type i cycles the envisaged saltern conditions were comparable in many ways to those prevailing during the deposition of halite cycles of the permian salado formation in new mexico and the permian san andres formation of the palo duro basin area in texas however in the canning basin the cycles are characterized by a much lower proportion of anhydrite implying perhaps a greater degree of continental restriction to the basin the moderately high level of bromine in the mallowa salt 156 5 43 5 ppm br for primary halite 146 1 54 7 ppm br for secondary halite accords with evolved continental brines although highly evaporative minerals such as polyhalite and magnesite are absent the bromine levels suggest little or no dissolution reprecipitation of primary halite and yet paradoxically there is little preservation of the primary depositional fabric the preservation of early halite cements and replacement textures supports the idea of an early shutdown of brine flow paths probably at burial depths of no more than a few metres and the resultant preservation of primary bromine values in the secondary halite

depositional environments of marine dominated bedded halite permian san andres formation texas the interpretation of the depositional environments in which bedded halite is formed is a frontier of evaporite sedimentology facies studies supplemented by geochemical data define an evaporite shelf depositional environment for the halite of the san andres formation of the palo duro basin texas panhandle that is significantly different from modern and previously described ancient halite precipitating environments such as playas salinas sabkhas and barred basins a coastal palaeoenvironmental setting for the san andres halite is defined by its facies position between normal marine shelf carbonates and aeolian deposits the san andres brine pool extended over more than 10 000 km2 but was very shallow perennial brine pool conditions thick sequences of halite alternated with ephemeral brine pool conditions halite haloturbated mudstone halite mudstone interbeds and microkarst the depositional sequence in the perennial brine pool includes 1 flooding and minor dissolution of previously deposited halite 2 precipitation of a thin lamina of gypsum and 3 evaporation and precipitation of halite bottom growth forms of halite including chevrons and vertically elongated crystals are well preserved cumulates of foundered floating crystals and rafts were probably abundant but have been almost completely recrystallized mudstone interbeds in halite and microkarst features cut into bedded halite record the episodic interruption of brine pool deposition and the establishment of subaerial conditions vertically elongated karst pits originate at the palaeosurfaces cross cut primary brine pool fabrics and are filled with geopetal sediment and coarse halite cement mudstone interbeds and mixtures of mudstone and halite have been haloturbated by the alternating growth and dissolution of halite reflecting conditions of fluctuating salinity the geochemistry of halite from both ephemeral and perennial brine pool environments reflects the frequent influx of marine brine indicating that communication with the marine environments to the south was good considering that restriction was sufficient to precipitate halite

nodular and enterolithic gypsum the sabkha tization of salt flat playa west texas abstract the sediment in the continental sabkha of salt flat west texas is typically millimeter laminated and composed of evaporitic and diagenetic minerals including gypsum dolomite halite calcite and aragonite these laminated evaporites were originally deposited on the floor of a late pleistocene perennial brine lake and now constitute the matrix of the modern sabkha sediments in the playa as climate changed from the more humid phase of the pleistocene to the more arid conditions of the holocene the lake dried up and these laminated evaporites were deflated into gypsum dunes or overprinted by a sabkha related hydrology near surface gypsum rich layers were transformed by capillary evaporation into coalescing nodular horizons at the same time in shallow vadose sediments less than 1 2 m subsurface the gypsum was dissolved and replaced either by halite or dolomite diagenetic gypsum was co precipitated as gypsum overgrowths and a patchwork of cement from a highly saline interstitial brine avg 156 000 mg l tds these processes of early diagenetic enlargement and coalescence of gypsum crystals created holocene gypsum nodules within the laminated host sediment diffuse growth of nodules in the lower vadose zone created chicken wire textures gypsum growth in the upper vadose zone was often concentrated along particular layers and formed enterolithic bands pleistocene lacustrine evaporites were in effect sabkhatized by a fall in the lake water level

cataclastic rocks along the nesodden fault oslo region norway a reactivated precambrian shear zone abstract the nesodden fault segment nfs is the northern segment of the oslofjorden fault of which separates the precambrian rocks in the east from palaeozoic rocks in the west the nfs is characterized by two groups of fault rocks group 1 comprises a series of crush breccias to ultracataclasites containing material derived from the precambrian basement these fault rocks were formed at a depth greater than 5 km by attrition brecciation and distributed crush brecciation at some time between the sveconorwegian orogeny 950 ma and the deposition of the cambrian alum shale 540 505 ma group 2 comprises five crush breccias with a similar quartz matrix containing fragments of older rocks and permian intrusive rocks they were formed at a depth of less than 3 5 km by the interaction of cataclasis and hydrothermal percolation attrition and implosion brecciation during the formation of the permo carbonifereous oslo rift 300 230 ma evidence of repeated hydrothermal fluid flow and fluidization has been recognized in the youngest breccias the seven different fault rocks emphasize the importance of fault reactivation and show that old fault zones commonly act as planes of weakness along which repeated deformation takes place and act as pathways for hydrothermal fluids

the hydrological setting occurrence and significance of gypsum in late quaternary salt lakes in south australia well exposed often laminated gypsum sequences occur in many quarternary salt lakes in southern south australia the gypsum in the salt lakes is classified by increasing grain size into gypsite gypsarenite and selenite the salt lakes are classified by age and hydrological setting into coastal salinas which are holocene sea water fed groundwater lakes and continental playas which are late pleistocene endorheic basins a study of the relationships between coastal salina hydrology and the associated gypsum deposition has shown the different types of gypsum form under distinct hydrological regimes as the hydrology above a coastal salina depositional surface changes through time so does the type of gypsum deposited application of a gypsum depositional model derived from a study of the coastal salina gypsum to those portions of a continental playa gypsum unit where deposition is no longer occurring confirms the applicability of the model to non salina gypsum deposits

sedimentology and evaporite genesis in a holocene continental sabkha playa basin bristol dry lake california bristol dry lake a 155 km2 continental sabkha playa basin in the mojave desert of south eastern california is filled with at least 300 m of interbedded terrigenous clastics gypsum anhydrite and halite evaporite facies conform approximately to a bull s eye pattern with gypsum and anhydrite surrounding a basin centre accumulation of halite transects through bristol dry lake from the alluvial fan to the centre of the playa reveal 1 crudely bedded alluvial fan clastics interfingering with 2 playa margin sand flat and wadi sand and silt followed by 3 gypsum anhydrite chaotic mud halite and clay of the saline mud flat and 4 salt pan halite beds terrigenous clastics were deposited in bristol dry lake by sheetflow and by suspension settling from ponded floodwater some sediment has been reworked by aeolian processes to form barchan dunes around the playa margin thin nodular like beds of anhydrite and several types of gypsum occur across most of the playa giant hopper shaped halite cubes are suspended in saline mud flat facies suggesting that they grew displacively in brine soaked sediment just below the surface thick beds 4 m of halite in the playa centre may have formed through a complex alternating history of subaqueous and intrasedimentary precipitation under the influence of periodic floods intense evaporation and brine level lowering and capillary discharge of brines the stratigraphy in the playa centre is cyclic an ideal cycle consists of 1 chaotic mud halite at the base overlain by 2 green to red clay with abundant giant hoppers and at the top 3 red clay gypsum and anhydrite with flaser to wavy bedded sand and silt this type of cycle probably records a gradual progradation of mud flat facies over salt pans bristol dry lake sediments are nearly identical to some of the permian evaporites of the permian basin region u s a and they can serve as modern analogues for ancient sabkha facies analysis

skagerrak graben the offshore continuation of the oslo graben abstract new seismic mapping of skagerrak reveals eight structural provinces in particular the skagerrak graben appears to be the continuation of the oslo graben the skagerrak graben is defined by two major late paleozoic faults trending northeast southwest connecting with the faults bounding the oslo graben extensive rotated block faulting is observed within the graben and a thickness of more than 2 s twt of lower paleozoic sediments has been preserved along the boundary faults sediments of probable late paleozoic age have been deposited as erosional products from the fault scarps and a central uplifted area a substantial part of these sediments may have a volcaniclastic origin the skagerrak graben continues to the north as a province of igneous rocks exposed at the sea floor with precambrian basement on either side the skagerrak graben is flanked by small half grabens containing probable early paleozoic sediments indicating that a wider area was involved in the rift process the formation of the skagerrak graben is related to the dextral strike slip movements along the sorgenfrei tornquist zone in late paleozoic time the kimmerian extensional movements along this zone indicate however a decoupling of the skagerrak graben at this time only minor inversion in the southwestern part of the skagerrak graben reflects the major inversion tectonism along the sorgenfrei tornquist zone in late cretaceous early tertiary times

metallogeny and lead isotope data from the oslo paleorift abstract the permian oslo paleorift situated at the southwestern margin of the baltic shield includes the oslo region and its southward extension under skagerak the exposed part of the paleorift comprises two half grabens vestfold and akershus graben segments filled with permian sediments volcanics and intrusive rocks as well as cambrc silurian sedimentary rocks the evolution of the oslo rift can be divided into a rifting stage a caldera stage and a batholith stage the rifting stage is characterized by weak mineralization of native copper in basalts the caldera stage includes fe ti cumulates in subvolcanic layered alkali gabbros nb ree disseminations in aphyric trachytes and epigenetic mo mineralizations the batholith stage includes porphyry and intraplutonic vein deposits of mo and contact metasomatic deposits of fe w mo be zn pb cu bi as and f along the rift margin in precambrian gneisses and amphibolites there are vein deposits of fe oxides base metals and native silver the relationship between the rift margin deposits and the evolution of the rift is still unclear but many deposits are spatially related to permian dolerite dykes lead isotope data from hydrothermal deposits in the oslo rift fall into two main groups the first group comprises the rift margin deposits they are very radiogenic and reflect an upper crustal source for the metals the source of the silver deposits generally has a lower value and a higher w value than the base metal deposits the rift margin deposits are therefore not a product of metal zonation from a single hydrothermal solution the second group comprises deposits associated with granitic intrusions in the oslo graben and reflects a lower to intermediate crustal source for the metals deposits in the vestfold graben segment are generally more radiogenic than deposits in the akershus graben segment and this difference may be explained by a small contribution of highly radiogenic lead from underlying precambrian granites the data show that individual intrusions can be characterized by their lead isotope composition and thus confirm the relationship between intrusions and mineralizations the data indicate a more radiogenic source for the peralkaline intrusions compared to the biotite granites but more data are needed to confirm this suggestion

paleostress reconstruction from kinematic indicators in the oslo graben southern norway new constraints on the mode of rifting abstract the oslo graben southern norway is a n s trending carboniferous permian rift system characterized by major mafic to silicic magmatism n s trending faults reactivation of preexisting precambrian faults and formation of half grabens magmatism is expressed by the presence of lavas dyke injections cauldron formation and the intrusion of batholiths paleostress analyses mainly based on slickensides have been performed in the area both within and outside the rift structure combined with the current tectono magmatic model of the oslo graben area the analyses show the following stress evolution from the caledonian orogeny with its compressional tectonics in the silurian to the late stages of the carboniferous permian rifting nw se compression occurred in silurian times due to the continental collision of the caledonian orogeny after a long period of a missing geological record the area was affected by n s compression during the late carboniferous a shallow sedimentary basin developed indicative for the pre and proto rift phases the transition from the proto rift phase to the initial rift phase is marked by a transition from a transpressional regime into a transtensional regime during the early permian the stress regime changed from pure extension to radial extension we suggest that the oslo graben can not simply be explained in terms of or passive or active rifting but that a combination of both evolving through time is more suitable to explain the observations we propose a model in which the oslo graben is initially triggered by far field stresses in the latest carboniferous and earliest permian which infers opening due to passive rifting these far field stresses are suggested to be linked to the hercynian orogeny active to the south in central europe in carboniferous times and the reorganisation of the pangea supercontinent in early permian times the stress regime caused radial extension indicative for uplift and a change to an active mode of rifting simultaneously with the radial extension large volumes of magma are emplaced at near surface levels despite all alternative models an anomalous thermal gradient is necessary to create these large volumes of magmatic material we propose therefore that shortly after the onset of rifting the dominant rifting mode becomes active plume related although far field stresses might still be present

paleobrine temperatures chemistries and paleoenvironments of silurian salina formation f 1 salt michigan basin u s a from petrography and fluid inclusions in halite abstract a 3 7 m stratigraphic section of the silurian salina group f 1 salt detroit salt company mine michigan basin was studied to document the paleodepositional conditions including brine chemistry and temperature of evaporite precipitation sedimentary features including rippled surfaces halite chevrons cumulate halite and dissolution surfaces confirm a shallow water depositional setting penecontemporaneous dissolution surfaces indicate that waters undersaturated with respect to halite were introduced into the michigan basin and that the basin brines were nonstratified during flooding allowing inflow waters to come into contact with halite crystals at the bottom fluid inclusions in primary chevron halite have homogenization temperatures between 5 and 25 c which represent brine temperatures during halite precipitation these temperatures are lower than expected for a marginal marine basin at a subtropical paleolatitude and they suggest a climatic cold spell for a period of thousands of years the major ions in fluid inclusion brines analyzed using the esem x ray eds method have relatively low concentrations of na mg2 and so42 and elevated concentrations of ca2 compared to modern seawater brine inclusion chemistries from the f 1 salt compare closely with the major ion composition of fluid inclusions from other silurian marine halites a 1 a 2 and b evaporites of the salina group and carribuddy group canning basin australia these results indicate a seawater parentage of the michigan basin f 1 salts and that the major ion chemistry of seawater remained more or less constant during the last 15 million years of the silurian detailed documentation of the temperatures brine chemistries and environments of halite deposition is a prerequisite for geobiological study of the entrapment of ancient microbes in salt the results reported here can help pinpoint the timing of entrapment of microorganisms in fluid inclusions in halite and can be used to argue that brine inclusions in the f 1 halites have been isolated and preserved in an unmodified state for more than 400 million years

crustal extension in the oslo graben se norway a method incorporating magmatism and erosion abstract three methods for determining crustal extension were examined for use in the late paleozoic oslo graben of southeast norway 1 1 analysis of faults and dikes 2 2 volume budget isostasy 3 3 analysis of crustal thickness distribution assuming constant crustal volume during extension these methods were found not to be suitable mainly due to rift age and magmatism instead a new integrated model which combines the effect of moho contours volumes of mantle injecta and erosion was used volumes of intracrustal mantle injecta are found from a model combining data on upper crustal felsic magmatic rocks with petrological and gravity inversion models for the vestfold graben segment in the south this implies twice the stretching estimate as compared to stretching derived from moho contours alone and two to three times the estimate from an inadequate set of fault and dike information the batholiths mark zones of high crustal thinning and magmatism was most likely caused by a combination of a thermal anomaly and lithospheric stretching

epitaxial salt reefs and mushrooms in the southern dead sea salt usually accumulates in shallow ephemeral brines as simple flat beds however in permanent brines deepening in solar evaporation ponds in the southern dead sea salt accumulation is complicated by the growth of isolated vegetable like salt structures

origin of depositional cycles in a permian saline giant the salado mcnutt zone evaporites of new mexico and texas two types of metre scale depositional cycles have been recognized within the mcnutt potash zone of the permian salado evaporites the cycles record progressive drawdown and concentration of brine in a shallow marginal marine basin type i cycles made of basal carbonate siliciclastic mudstone then anhydrite polyhalite pseudomorphous after primary gypsum overlain by halite and capped by muddy halite are interpreted as marine sea water dominated they formed by massive spillover of sea water through a connection with the permian ocean at high relative sea level stands lagoonal mudstone and anhydrite polyhalite after subaqueous gypsum followed by relative lowering of sea level and gradual basin restriction shallow saline lake and salt pan halite and ending with a desiccated basin isolated from the permian ocean salt pan halite and saline mudflat muddy halite cycle cap type ii cycles occur between the type i cycles and contain halite overlain transitionally by muddy halite they record a temporal evolution of environments from a shallow saline lake to an ephemeral salt pan saline mudflat complex and are interpreted as continental dominated sequences sourced by meteoric inflow from surrounding land areas that mixed with variable amounts of sea water either residual or introduced into the salado basin by seepage the vertical stacking of type i cycles is best explained by periodic invasions of sea water into the salado basin coincident with eustatic sea level rises the continental dominated type ii cycles formed during intervening periods of eustatic sea level fall and low stands the maximum time interval between major marine incursions is an average of 10 5 yr

deposition of a saline giant in the mississippian windsor group nova scotia and the nascent late paleozoic ice age abstract saline giants are vast marine evaporite deposits that currently have no modern analogues and remain one of the most enigmatic of chemical sedimentary rocks the mississippian windsor group ca 345 ma maritimes basin atlantic canada is a saline giant that consists of two evaporite rich sedimentary sequences that are subdivided into five subzones sequence 1 is composed almost entirely of thick halite belonging to subzone a osagean sequence 2 is in unconformable contact and comprised of stacked carbonate evaporite peritidal cycles of subzones b through e meramecian subzone b the focus of research herein documents the transition from wholly evaporitic to open marine conditions and thus preserves an exceptional window into the processes forming saline giants lithofacies stacking patterns in subzone b reveal that higher order fluctuations in relative sea level produced nine stacked parasequences interpreted to reflect high frequency glacioeustatic oscillations during the onset of the late paleozoic ice age each parasequence reflects progradation of intertidal and sabkha sediments over subtidal carbonate and evaporite deposits dissimilarities in cycle composition between sub basins imply the development of contrasting brine chemistries from differing recharge rates with the open ocean what the windsor group shows is that evaporite type is ostensibly linked to the amplitude and frequency of sea level rise and fall during deposition true saline giants like the basinwide evaporites of sequence 1 apparently require low amplitude long frequency changes in sea level to promote the development of stable brine pools that are only periodically recharged with seawater by contrast the high amplitude short frequency glacioeustatic variability in sea level that controlled the accumulation of peritidal evaporites in subzone b produce smaller subeconomic deposits with more complex facies relationships

synsedimentary dissolution pits in halite of the permian salado formation southeastern new mexico abstract mining operations in the permian salado formation for the waste isolation pilot plant wipp reveal filled pits in halite beds which are interpreted as synsedimentary dissolution features the pits range from a few centimeters to as much as 1 m wide and 1 m deep and they clearly truncate thin underlying beds halite hopper crystals and possible chevron halite structures occurring in the pits and continuous undeformed bedding in overlying units demonstrate that the pits formed and filled before deposition of the overlying unit pseudomorphs of halite after gypsum swallow tail crystals are part of the evidence that the dissolution pits formed in shallow water

physical characteristics and evolutionary trends of continental rifts rifts may be defined as zones beneath which the entire lithosphere has ruptured in extension they are widespread and occur in a variety of tectonic settings and range up to 2 600 m y in age the object of this review is to highlight characteristic features of modern and ancient rifts to emphasize differences and similarities in order to help characterize evolutionary trends to identify physical conditions favorable for initiation as well as termination of rifting and to provide constraints for future modeling studies of rifting rifts are characterized on the basis of their structural geomorphic magmatic and geophysical features and the diverse character of these features and their evolutionary trends through time are discussed mechanisms of rifting are critically examined in terms of the physical characteristics and evolutionary trends of rifts and it is concluded that while simple models can give valuable insight into specific processes of rifting individual rifts can rarely if ever be characterized by well defined trends predicted by these models more data are required to clearly define evolutionary trends and the models require development to incorporate the effects of lithospheric heterogeneities and complex geologic histories

the oslo graben gravity high and taphrogenesis abstract the paleo oslo rift oslo graben system is geophysically characterized by a pronounced gravity high a reinterpretation of gravity data in the region using a flexible inversion scheme indicates that the causative body has a density contrast of 0 06 g cm 3 the inversion results also suggest that this body is most likely located in the upper part of the lower crust and extends eastward well outside the graben area proper estimates of crustal thicknesses before and after the rifting aborted suggest that rifting commenced in the south and then progressed northwards the corresponding pole of rotation was located approximately 240 km nne of the city of oslo the hypocentres of 7 small local earthquakes were found to be located on the periphery of the anomalous body as derived from gravity modeling seismic investigations including 3 d imaging using norsar data have failed to provide conclusive evidence on intrusion of asthenospheric material in the lithosphere as part of the oslo graben taphrogenesis the explanation proposed here is that the geophysical imprints of oslo rift oslo graben evolutionary processes within the upper mantle have been considerably weakened over the associated time span of some 150 200 ma finally it is hypothesized that the asthenospheric injection into the lithosphere took place over a relatively small cross sectional area of the moho the further ascent of the gabbroic magma was partly hindered by the relatively rigid upper crust thus causing extensive lateral flow eastwards outside the oslo graben proper in the 10 20 km depth range

formation of the steer s head geometry of sedimentary basins by differential stretching of the crust and mantle if the lithospheric mantle is stretched over a wider region than the crust postrift stratigraphic onlap will occur at basin margins giving rise to a steer s head geometry space problems are avoided by making the total amount of extension in the lithospheric mantle equal to that in the crust the predicted pattern of onlap agrees well with that observed if the lithospheric mantle is stretched over a fractionally wider region than is the crust thus extensive stratigraphic onlap can be produced easily without recourse to sea level rise or flexural rigidity

evaporites marine or non marine division of evaporites into those types that retain information about primary mineralogy textures and structures primary evaporites and those types so extensively altered that few or no primary features remain secondary evaporites only primary evaporites can give unequivocal data bout their marine or non marine origin to help evaluate such primary deposits they have been further divided using the following attributes 1 whether the depositional setting was marine or non marine 2 whether the source water was a seawater b meteoric c hydrothermal d diagenetic e volcanogenic or f mixed specific criteria for making distinctions are 1 kinds of fossils 2 sedimentology of the associated non saline facies 3 kinds of primary saline minerals 4 association of such saline minerals both in qualitative and quantitative terms 5 trace element isotope and fluid inclusion geochemistry of such saline minerals modified journal abstract

relation between continental rifting and uplifting evidence from the suez rift and northern red sea abstract the suez rift and the northern red sea are flanked by high standing shoulders with elevations often exceeding 2 km and with a structural relief reaching 4 5 km in this area pre rifting erosion was small not more than a few hundred meters there is no evidence that a precursory uplift formed along the site of future rifting uplifting and erosion began essentially at the same time as rift faulting and magmatism in late oligocene and early miocene limes in the gebel serbal area sinai where the uplifting history is resolved best the tectonic relief reached about 800 m by the end of the early miocene and a similar amount of tectonic uplifting developed in later times other parts of the rift shoulders probably had a similar uplifting history but the total uplifting was often less considerable erosion in places more than 3 km occurred mainly since middle miocene times and caused considerable isostatic rebound this magnified the uplifting and caused the structural and topographic relief to be considerably greater than the tectonic uplifting initial uplifting and contemporaneous magmatic activity were probably caused by rising of deep hot material which thinned the lithosphere and heated it such deep seated processes probably continued to operate also during the later stages of tectonic uplifting though igneous activity no longer occurred on the surface stretching of the lower lithosphere under the rift shoulders could have also contributed to their uplifting the features of uplifting history of the studied region may well be typical of other active rifts

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 morton black hypothesis for the thinning of continental crust revisited in western afar abstract geological observations along the western margin of afar show that the morton black model in which thinning of upper continental crust at a nascent continental margin is accomplished through block faulting and tilting is not wholly applicable rather dikes are concentrated into swarms suggestive of important direct dilatation faults are concentrated into sets within which no block tilting steeper than 45g is observed therefore attenuation of the afar margins requires a significant role for processes other than block tilting and in some margin sectors the transition from continental to neo oceanic crust may be relatively abrupt

danish basin subsidence by triassic rifting on a lithosphere cooling background i report here the first quantitative study of the entire post palaeozoic subsidence history in the ensemble of north west european sedimentary basins data obtained from wells suggests that the late permian through mesozoic subsidence of the danish basin is a result of lithosphere wide cooling following an igneous event of late carboniferous to early permian age this cooling subsidence was superposed by major triassic rifting and minor mid jurassic uplift both events apparently not having significant thermal consequences the danish basin did not experience the late jurassic to early cretaceous rifting and subsequent cooling which were so important in the development of the central north sea rifts this result suggests that a reappraisal of north sea tectonics should look for pre jurassic subsidence

three dimensional geophysical characterization of the la rambla and zafra de zancara anticlines loranca basin central spain this study has been supported by the instituto para la reestructuracion de la mineria del carbon y el desarrollo alternativo de las comarcas mineras ministry of energy and industry spain

milankovitch band cyclicity in bedded halite deposits contemporaneous with late ordovician early silurian glaciation canning basin western australia abstract the mallowa salt of the early palaeozoic carribuddy group canning basin western australia is a halite mudstone anhydrite dolomite evaporite sequence of late ordovician to early silurian age deposited in barred marginal marine to ephemeral saltpan and saline mudflat environments bhp utah minerals potash exploration well gingerah hill no 1 obtained drill core of a 477 m stratigraphic interval of the mallowa salt which permitted the study of geochemical variations in a thick halite sequence contemporaneous with late ordovician early silurian glaciation on other continents a strip of uniform width was continuously ground from the core and the powdered rock bagged at 1 m intervals for geochemical assay the resulting geochemical stratigraphic series display conspicuous cyclicity several orders of cycles ranging from 3 m to100 m stratigraphic thickness are discernible in the plots of minor constituents br mgo and k2o in extracted soluble salts and also in the plots of na2o and total salts content fast fourier transform fft of the br mgo and k2o series gives principal spectral peaks in the 0 00 0 10 cycles m frequency band at 250 113 35 4 22 1 and 19 7 m as well as peaks at 65 m and between 2 8 and 13 5 m fft of the na2o and total salts series shows dominant spectral peaks at 220 259 and 109 114 m if the strongest spectral peak at 113 m is taken to represent the relatively stable eccentricity period of 100 ka and a constant net rate of accretion assumed for long sections the other main periods in the mallowa salt as exemplified by the br spectrum would be31 3 3 0 19 6 1 1 and17 4 1 1 ka these figures are consistent with predicted late ordovician early silurian 440 ma periods for obliquity 30 5 ka and precession 19 3 and 16 4 ka based on evolutionary change in the earth moon system additional spectral peaks identified with implied periods of approximately 206 233 57 5 8 12 and 2 5 3 5 ka also may be of palaeoclimatic relevance the relative amplitudes and structure of spectral peaks in the 0 00 0 10 cycles m frequency band support the identification of climatic oscillations forced by orbital cycles the data indicate a precession eccentricity dominated pattern which accords with the know low palaeolatitude

surface deformation in volcanic rift zones abstract the principal conduits for magma transport within rift zones of basaltic volcanoes are steeply dipping dikes some of which feed fissure eruptions elastic displacements accompanying a single dike emplacement elevate the flanks of the rift relative to a central depression concomitant normal faulting may transform the depression into a graben thus accentuating the topographic features of the rift if eruption occurs the characteristic ridge trough ridge displacement profile changes to a single ridge centered at the fissure and the erupted lava alters the local topography a well developed rift zone owes its structure and topography to the integrated effects of many magmatic rifting events to investigate this process we compute the elastic displacements and stresses in a homogeneous two dimensional half space driven by a pressurized crack that may breach the surface a derivative graphical method permits one to estimate the three geometric parameters of the dike height inclination and depth to center and the mechanical parameter driving pressure rock stiffness from a smoothly varying displacement profile direct comparison of measured and theoretical profiles may be used to estimate these parameters even if inelastic deformation notably normal faulting creates discontinuities in the profile geological structures open cracks normal faults buckles and thrust faults form because of stresses induced by dike emplacement and fissure eruption theoretical stress states associated with dilation of a pressurized crack are used to interpret the distribution and orientation of these structures and their role in rift formation

textural features of holocene perennial saline lake deposits of the taoudenni agorgott basin northern mali abstract the holocene salt lake deposits of the taoudenni agorgott basin northern mali mainly consist of sediments with a high glauberite na2ca so4 2 content the remainder of the deposits largely consists of salt beds with a bloedite na2mg so4 2 4h2o thenardite na2so4 or halite nacl composition a petrographical study of the deposits demonstrates that they formed in a perennial lake that experienced a gradual decrease in water depth textural features of the glauberite dominated deposits are found to be related to water depth through the control that this factor exerts on the sensitivity of the lake to changes in water supply and to short term variations in evaporation rates in this way layering due to variations in glauberite content and crystal size is inferred to be typical of deposits that formed in shallow water whereas unstratified deposits are the product of high lake level stages halite textures are found to be indicative of the place within the water column where crystal growth occurred along the lake bottom or higher which is mainly determined by water depth and partly by evaporation rates the oldest halite beds are largely unaltered cumulate deposits whereas the youngest layers developed exclusively through bottom growth the basal part of one thick halite bed at a level between these two groups of halite layers developed by an alternation of both types of growth in response to variations in evaporation rates variations in mineralogical composition between and within the salt beds that formed during the earliest periods with a higher salinity up to the first stage with halite formation record a change in lake water chemistry with time but they are in one instance also determined by an early diagenetic mineral transformation

skagerrak evolution derived from tectonic subsidence abstract stratigraphic data gathered from 32 points in the continental skagerrak basin have been analyzed by basin modelling methods non tectonic effects of sediment loading were removed by backstripping giving the observed tectonic subsidence this was compared to synthetic subsidence curves constructed on the basis of a combined lithospheric extension and metamorphism model where gabbroic magma which was emplaced during the permian is transformed to granulits eclogite facies the overall picture is that prior to 230 ma subsidence was roughly similar north and south of the fennoscandian border zone fbz after this time the northern area sagged due to the still preserved lithospheric thermal anomaly and continued metamorphism whereas the southern area was also affected by extension a physical explanation for this contrast in evolution is given by considering the relative lithospheric strength history of the areas the northern area becoming progressively stronger with time relative to the southern area as crustal thicknesses are reduced total post 570 ma extension is approximately 1 4 40 north of the fbz and 1 5 50 south of it north of the fbz the average thickness of the metamorphic layer is between 3 and 7 km whereas south of the zone a thickness of 1 2 km is estimated this difference may reflect variations in the degree of permian magmatism the temperature history of the sediments and the degree of organic maturation has been calculated by the tti method the youngest sediments to have experienced temperatures high enough for significant oil generation are estimated to be of bathonian age the present model heat flow of about 57 mw m 2 is in agreement with observations lateral displacements along the fbz are of the order of 10 20 km and have consequently only played a minor role in the development of the skagerrak basin a thermal lithosphere thickness of about 125 km is inferred