structural analysis of a transform fault rift zone junction in north iceland abstract on the north coast of iceland the rift zone in north iceland is shifted about 120 km to the west where it meets with and joins the mid ocean kolbeinsey ridge this shift occurs along the tjornes fracture zone an 80 km wide zone of high seismicity which is an oblique non perpendicular transform fault there are two main seismic lineaments within the tjornes fracture zone one of which continues on land as a 25 km long wnw trending strike slip fault this fault referred to as the husavik fault meets with and joins north trending normal faults of the theistareykir fissure swarm in the axial rift zone the most clear cut of these junctions occurs in a basaltic pahoehoe lava flow of holocene age where the husavik fault joins a large normal fault called gudfinnugja at this junction the husavik fault strikes n55 w whereas gudfinnugja strikes n5 e so that they meet at an angle of 60 the direction of the spreading vector in north iceland is about n73 w which is neither parallel with the strike of the husavik fault nor perpendicular to the strike of the gudfinnugja fault during rifting episodes there is thus a slight opening on the husavik fault as well as a considerable dextral strike slip movement along the gudfinnugja fault consequently in the holocene lava flow there are tension fractures collapse structures and pressure ridges along the husavik fault and pressure ridges and dextral pull apart structures subparallel with the gudfinnugja fault the 60 angle between the husavik strike slip fault and the gudfinnugja normal fault is the same as the angle between the tjornes fracture zone transform fault and the adjacent axial rift zones of north iceland and the kolbeinsey ridge the junction between the faults of husavik and gudfinnugja may thus be viewed as a smaller scale analogy to the junction between this transform fault and the nearby ridge segments using the results of photoelastic and finite element studies a model is provided for the tectonic development of these junctions the model is based on an analogy between two offset cuts mode i fractures loaded in tension and segments of the axial rift zones or parts thereof in the case of the husavik fault the results indicate that the tjornes fracture zone in general and the husavik fault in particular developed along zones of maximum shear stress furthermore the model suggests that as the ridge segments propagate towards a zero underlapping configuration the angle between them and the associated major strike slip faults gradually increases this conclusion is supported by the trends of the main seismic lineaments of the tjornes fracture zone

rift transform junction in north iceland rigid blocks and narrow accommodation zones revealed by gps 1997 1999 2002 summary the current tectonics of north iceland are characterized by rifting episodes along fissure swarms and by ml 6 7 earthquakes in the transform zone between the northern rift zone and the kolbeinsey ridge north of iceland the last rifting period 1975 1984 was associated with an average opening of 5 m along the krafla fissure swarm post rifting deformation has been revealed by gps investigations in northern iceland a gps network was occupied in 1997 1999 and 2002 to quantify present day displacements and their variation with time on

chemical influence on α recoil damage annealing in apatite implications for u th he dating article apatite u th he ahe ages reflect the influence of several parameters such as grain size thermal history and diffusion kinetics previous work has suggested that recoil damage produced by the u th decay chains can have a significant effect on he diffusion kinetics models taking into account damage production and annealing via a parameterization similar to that for apatite fission tracks aft have been proposed comple mentary to these models in this work we demonstrate that grain chemistry influences the annealing rate of diffusivity altering damage in the same manner as for aft and thus is not constant across all apatites the key parameter of the annealing law rmr0 is examined and its theoretical impact on ahe age is tested in cer tain situations the ahe age can be significantly affected by changes in the damage annealing rate for sam ples having undergone a long stay in the ahe partial retention zone 50 80 c ahe ages can diverge by more than 15 over a reasonable range of annealing rates in addition this hypothesis is tested on a geolog ical case well samples of triassic sandstones at 67 5 1 5 c at depth in the paris basin france aft data and ahe ages have been measured on detrital apatite in addition to chemical analysis of the grains dated for aft ages for both thermochronometers present similar histogram distributions with younger ages for ahe aft 14 208 ma n 117 ahe 4 76 ma and one grain at 120 ma n 36 the apatite grains are character ized by dpar values from 1 2 m to 2 9 m and cl content ranging from 0 05 to 0 9 wt using a thermal his tory reconstruction based on aft data and geological evidence it is shown that the non correlation observed between ahe age and eu content can be simply explained by changing the rmr0 value we conclude that to fully access the ahe age meaning in terms of temperature sensitivity the parameters of the annealing law stemming from the grain chemistry need to be considered

seismic risk at the rift transform junction in north iceland the current tectonics of north iceland are characterised by max m 6 7 earthquakes in the transform zone between the northern rift zone and the kobleinsey ridge the last rifting period 1975 1984 was associated with an opening of up to 9 m along the krafla fissure swarm this period was followed by the disappearance of microseismicity along the eastern part of husavik flatey fault the main structure of the transform zone a gps network was occupied in 1995 and again in 1997 to study present day deformation on both sides of the on land part of hff and around its junction with the northern rift zone the main deformational features observed are 1 in tjornes the hff is locked above the ductile brittle boundary below which ductile creep is occurring and 2 aseismic opening along the theystareykir and krafla fissure swarms and along a n 120 e superficial fault that marks the southern boundary of tjornes

how do asperities fracture an experimental study of unbroken asperities abstract the fracture of a shear fault containing several unbroken asperities in a granitic porphyry is examined in detail via acoustic emission ae data collected by a high speed multi channel waveform recording system the experimental results reveal that a quasi static nucleation of the shear faulting corresponds to the fracture of coupled asperities on the fault plan aes caused by the fracture of individual asperities exhibit similar characteristics to the sequence for natural earthquakes including foreshock mainshock and aftershock events foreshocks initiated at the edge of the asperity occur with an event rate that increases according to a power law of the temporal distance to the mainshock and with a decreasing b value from 1 1 to 0 5 one or a few mainshocks then initiate at the edge of the asperity or the front of the foreshocks the aftershock period is characterized by a remarkable increase and subsequent gradual decrease in b value and a decreasing event rate obeying the modified omori law which has been well established for earthquakes the fracture of neighboring asperities is then initiated after the mainshock of a particular asperity presumably due to redistribution of the strain energy accumulated within an asperity which is released by the mainshock resulting in enhancement of the stress concentration around the nearest neighboring intact asperities the progressive fracturing of multiple coupled asperities during the nucleation of shear faulting results in short term precursory fluctuations in both b value and event rate which may prove useful information in the prediction of failure of the main fault plane of seismic earthquakes

determination of maximum paleotemperatures of burial mptb of sedimentary rocks from pyrolysis data on the associated organic matter basic principles and practical application the balazuc 1 borehole drilled in the frame of the geologie profonde de la france programme and the work carried out by various specialists provided the samples and the independent paleotemperature assessment data both needed to check a model aiming to the straightforward determination of maximum paleotemperatures of burial mptb from rock eval pyrolysis data basically the model supposes that laboratory e g rock eval pyrolysis resumes the successive elimination of kerogen moieties of increasing thermal stability where it had stopped during burial diagenesis then the temperature tmin determined at the onset of the s2 rock eval pyrolysis peak supposed to correspond to a peculiar kerogen moiety which was just undergoing cracking when maximum burial depth was reached might be used to calculate the corresponding mptb taking into account the different values of the experimental and natural thermal gradients the good agreement between calculated mptb values and results of fission track and fluid inclusion studies carried out on balazuc 1 borehole samples supports the validity of the model according to fundamental considerations the application of this approach is limited to samples having at least reached the onset of the metagenetic stage other possible limitations and the causes of uncertainty on the obtained results are discussed a detailed operating procedure is presented

viscosity structure beneath northeast iceland the dynamics of crustal rifting in iceland depend strongly on the lower crustal rheology which controls the intensity of upper crustal stress concentration and scale time of heat diffusion from the underlying mantle plume while magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fraction of partial melt observations of low seismic attenuation and strong shear wave transmission suggest a much cooler lower crust and upper mantle since viscosity is also sensitive to the degree of partial melt present viscosity estimates for these regions could shed light on the factors responsible for these observations in this study we utilize horizontal and vertical displacement vectors determined in gps campaigns in northeast iceland since 1986 these are modeled in terms of steady state tectonic loading plus postseismic postdiking relaxation following the 1975 1984 krafla rifting episode as first proposed by foulger and others with the elastic part of the model fixed by external constraints these data have a high sensitivity to the viscosity structure beneath iceland lower crust and upper mantle viscosities of about 3 1019 pa s and 3 1018 pa s respectively yield the closest agreement with the data our lower crustal viscosity estimate is consistent with the low attenuation and low subsolidus temperature for the lower crust inferred in recent studies inversions for fissure opening during the krafla rifting episode yield about 7 m of opening centered on the krafla rift as is observed allowing for contemporaneous deep rifting on vertical faults along the askja segment partially accounts for the observed increase in separation across the rift during 1987 1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the askja rift during 1987 1990 recent deep normal faulting beneath the askja rift and further south might explain all of these remaining features

environmental signal propagation in sedimentary systems across timescales abstract earth surface processes operate across erosionally dominated landscapes and deliver sediment to depositional systems that can be preserved over a range of timescales the geomorphic and stratigraphic products of this source to sink sediment transfer record signals of external environmental forcings as well as internal or autogenic dynamics of the sedimentary system here we evaluate environmental signal propagation across sediment routing systems with emphasis on sediment supply qs as the carrier of up system forcings we review experimental numerical and natural examples of source to sink sediment routing and signal propagation during three timescales 1 historic which includes measurement and monitoring of events and processes of landscape change and deposition during decades to centuries 2 centuries to several millions of years referred to as intermediate timescale and 3 deep time we discuss issues related to autogenic dynamics of sediment transport transient storage and release that can introduce noise lags and or completely mask signals of external environmental forcings we provide a set of conceptual and practical tools for evaluating sediment supply within a source to sink context which can inform interpretations of signals from the sedimentary record these tools include stratigraphic and sediment routing system characterization sediment budgets geochronology detrital mineral analysis e g thermochronology comparative analog approaches and modeling techniques to measure calculate or estimate the magnitude and frequency of external forcings compared to the characteristic response time of the sediment routing systems

anti torque systems of electromechanical cable suspended drills and test results abstract to prevent spinning of the upper non rotated part of the electromechanical drill an anti torque system has to be included in the downhole unit at the same time the anti torque must allowthe drill to move up and down the borehole during drilling and tripping operations usually the anti torque system has a blade form of various designs that engages with the borehole wall and counteractsthe torque from the stator of the driving motor this paper presents a review of the different anti torquesystems and test results with selected designs leaf spring skate and u shaped anti torque systems experiments showed that the skate anti torque system can provide the maximal holding torque between67 and 267 n m 1 depending on the skates outer diameter and ice temperature while the leaf springanti torque system can provide only 2 5 40 n m 1 in case of straight contact between the ice and theleaf springs the total resistance force to axial movement of the skate anti torque system lies in therange 209 454 n if the system is vibrating for the leaf spring anti torque system the total axialresistance force is far less 19 243 n keywords

sm nd dating of fluorite from the worldclass montroc fluorite deposit southern massif central france a three point sm nd isotope isochron on fluorite from the very large montroc fluorite vein deposit southern massif central france defines an age of 111 13 ma initial end of 8 6 and initial 87sr 86sr of 0 71245 suggest an upper crustal source of the hydrothermal system in agreement with earlier work on fluid inclusions which indicated a basinal brine origin the mid cretaceous age of 111 ma suggests the albian aptian transition as the most likely period for large scale fluid circulation during a regional extensional tectonic event related to the opening of the north atlantic ocean

ocean ridge discontinuities in iceland abstract the tjornes fracture zone in north iceland and the south iceland seismic zone are ocean ridge discontinuities that are partly exposed on land these are zones of high seismicity but contrasting style of crustal deformation detailed field studies and numerical modelling suggest that the tjornes fracture zone has all the characteristics of an oceanic transform fault is 7 9 ma old and developed along the zone of maximum shear stress between the nearby tips of the rift zones of the kolbeinsey ridge and north iceland conversely the results indicate that the south iceland seismic zone developed within an area of shear stress concentration between the overlapping rift zones in south iceland the development of the south iceland seismic zone is a consequence of the propagation of the eastern rift zone to the southwest during the past 3 ma

thermal maturity assessment study of the late pliensbachian early toarcian organic rich sediments in southern france grands causses quercy and pyrenean basins abstract thermal maturity of late pliensbachian early toarcian organic rich sediments in the grands causses sueges section quercy caylus section and pyrenean pont de suert section basins was determined through multiple parameters including spore coloration index sci hydroid random reflectance hrr and spectral fluorescence of tasmanites algae max the main objective of this study is to test the effectiveness and make comparisons of organic matter thermal maturity in these three sections by different techniques and particularly hydroid reflectance for the sueges section sci presented a value of 3 5 4 0 hrr ranges between 0 36 and 0 47 which corresponds to 0 45 0 52 vreq and shows a good correlation with sci values spectral fluorescence analysis presents a max of 560 nm for most samples the fluorescence spectral maximum seems to be redshifted in comparison to other thermal maturity parameters the caylus section exhibited sci of 3 5 4 0 hrr ranges between 0 32 and 0 50 corresponding to 0 42 0 54 vreq spectral fluorescence analysis points out a max of 560 nm displaying a good correlation with sci when req values determined through sci are compared with equivalent vitrinite reflectance values determined through max a redshift of the fluorescence spectrum maximum is detected for the pont de suert section sci ranges between 8 0 8 5 and 8 5 9 0 hrr varies between 0 85 and 1 18 which corresponds to 0 76 0 98 vreq even though there are lower values than those obtained by sci vreq is reliable and acceptable to determine the thermal evolution stage furthermore hrr values also present a good correlation with sci these data indicate that the majority of the samples from the sueges and caylus sections are in the immature to early mature evolution stages and pont de suert section samples are in late mature to early overmature evolution stages for liquid hydrocarbons generation supporting a very similar thermal evolution for the grands causses and quercy basins and an entirely different thermal history for the pyrenean basin this is in accordance with the similar tectonic and sedimentary context of the quercy and grands causses basins the pont de suert section located near the collision front of the pyrenees mountains records a more complex sedimentary and tectonic history in addition this study presents the first reflectance data for jurassic hydroids which show a good correlation with other rank parameters

thermal history constraints from studies of organic matter clay minerals fluid inclusions and apatite fission tracks at the ardeche paleo margin ba1 drill hole gpf program france abstract the thermal history of the ardeche paleo margin southeastern france has been reconstructed using various analytical data from minerals and organic matter and conventional interpretation these methods comprise tmax and vitrinite reflectance measurements on organic matter determination of the smectite content in mixed layer illite smectite fluid inclusion microthermometry in quartz barite dolomite and anhydrite and fission track analyses in apatite the balazuc ba1 drill hole 1730 m deep intersects mainly jurassic carbonate rocks triassic sandstones and sulfate rich claystones and ends in carboniferous rocks the present bottom hole temperature is 70 c other drill holes less than 6 km away were studied when necessary one of the main objectives of this paper is to ssess the agreement between the different geothermometric methods and to check their limitations and convergence when used on carbonate and sandstone dominated reservoirs in the triassic sandstones primary aqueous inclusions in anhydrite cement and at the boundary between the detrital quartz grain and the quartz overgrowth are one phase brine inclusions entrapped below 70 c the homogenization temperature determined on the later barite cement is about 100 c aqueous two phase inclusions in healed microfractures in anhydrite are present at the base of the drill core near major faults temperatures as high as 210 c recorded for these fluid inclusions could be associated with episodic fluid injections from below through the fault zones during the early jurassic the fluid inclusions in dolomite most of them reequilibrated during burial indicate a trapping temperature of 130 145 c at 1600 m and a temperature decrease towards the surface this value is in good agreement with the mptb maximum paleotemperature of burial method on organic matter which converges to a maximum burial temperature of 130 c at the same depth the apatite fission track data agree with these temperatures and indicate that the temperature decreased below 120 10 c during the eocene these paleotemperatures much higher than the present ones can be explained by the erosion of 1900 m of mainly cretaceous sediments this interpretation implies a high average sedimentation rate during the cretaceous in agreement with data determined on a regional scale a time temperature evolution is proposed for the formations present in the balazuc drill hole and the eroded sedimentary rocks the application of the dual reaction model of the transformation of smectite to illite velde and vasseur 1992 shows that the data are in good agreement with the kinetic model however the vitrinite reflectance variation trend with depth predicted from the burnham and sweeney model 1989 is different from the data variation no definitive explanation can be proposed for the failure of aturity assessment through vitrinite reflectance in the ba1 drill hole

the ardeche palaeomargin of the south east basin of france mesozoic evolution of a part of the tethyan continental margin geologie profonde de la france programme abstract this paper incorporates the results of seismic and gravity investigations borehole data and studies of sedimentology biostratigraphy sequence stratigraphy and structural geology carried out in the framework of the deep geology of france programme the main purpose of the paper is to define the structure and to reconstruct the evolution of the ardeche palaeomargin of the south east basin of france from the stephanian to the end of the mesozoic the acquisition and processing of three seismic reflection profiles the drilling of two scientific boreholes ba1 and mm1 and subsequent structural modelling with the use of cross section balancing techniques applied to both the depth migrated seismic and geological sections provide new geometrical constraints which lead to the following hypothesis of the geodynamic evolution of the margin 1 1 late palaeozoic extension with the development of high angle basement faults asymmetrical permian half grabens pre extension sedimentary cover 2 2 during the middle triassic a pre rift stage with the creation of small fault blocks and in the late triassic the formation of an e w flexure 3 3 end lias early jurassic to middle jurassic formation of a major detachment in the carboniferous coal measures leading to complete decoupling of the mesozoic sedimentary cover from the basement this episode of extension coeval with the opening of tethys can be related to a phase of episodic rifting between the late sinemurian and the late bathonian i e during 30 ma 4 4 late jurassic to early cretaceous thermal subsidence with regional downwarping of the basement towards the east 5 5 after the late cretaceous the period of maximum burial pyrenean and alpine deformation in the south and east causing progressive uplift of the ardeche palaeomargin as much as two kilometres of mesozoic deposits subsequently being eroded from the section during the tertiary

widening of the krafla fissure swarm during the 1975 1981 volcano tectonic episode frequent distance measurements across the krafla fissure swarm north iceland recorded the extension accompanying the sequence of rifting events which started in december 1975 and lasted for 6 years an 80 to 90 km long section of the fissure swarm extended during this sequence of rifting and volcanic events maximum widening of about 8 m occurred 10 to 12 km north of leirhnjukur which is located above the center of the krafla magma reservoir from that location the amount of widening decreased north wards and is estimated to exceed 2 m where the seismicity indicated the northern termination of the present rifting off the north coast about 70 km north of leirhnjukur the amount of widening also decreased southwards and approached zero at 15 to 20 km south of leirhnjukur the ground deformation associated with these rifting events can be summarised as a narrow strip 1 to 2 km wide along the fissure swarm is heavily fractured with numerous open cracks parallel to the fissure swarm this fractured strip has subsided 2 to 3 m relative to its flanks the flanks of the fractured zone have been uplifted relative to regions farther away the uplift is not well constrained but tilt observations at several locations indicate about 1 m uplift the flanks of the rift zone have contracted perpendicular to the fissure swarm the maximum contractional strain exceeds 300 mm per km the amount of areal expansion windening of the fissure swarm times the length of the fractured zone associated with these rifting events is estimated to be about 0 30 km2 for individual events the area of expansion has been roungly proportional to the volume of subsidence above the krafla magma reservoir if the width of a new dike is equal widening of surface fissures the ratio of the subsidence volume to the area of expansion for the best observed events indicated a height of a new dike system as 2 4 to 2 8 km this ratio is significantly less for events of large lava production but even during these events the majority of magma leaving the krafla reservoir was apparently emplaced in subsurface fissures

fluid overpressure and stress drop in fault zones using the average aspect length maximum thickness ratios of 379 mineral filled extension mode i veins from an active fault zone the fluid overpressure during their development with reference to the minimum compressive principal stress 3 is estimated at 20 mpa emplacement of such veins increases 3 and can generate a temporary stress barrier to the propagation of subsequent hydrofractures on meeting a subhorizontal stress barrier vertically propagating hydrofractures may change into water sills where the fluid pressure is at or above lithostatic in this model stress barriers and thus water sills can form at any depth in and in any type of fault zones for such a high fluid pressure the product of the coefficient of sliding friction and the normal stress in the modified griffith criterion becomes essentially zero and the driving stress associated with faulting equal to twice the in situ tensile strength of the host rock for typical in situ tensile strengths of 2 3 mpa the driving stresses for slip on overpressured fault planes is 4 6 mpa these results are in good agreement with the commonly measured average static stress drops of 3 6 mpa during earthquakes

effect of alpha damage annealing on apatite u th he thermochronology recent data on he diffusion challenge the temperature sensitivity of apatite u th he thermochronology the damage induced by recoil of u and th decay series during emission of particles recoil damage has been proposed to modify he diffusion properties through time however we propose that annealing of these irradiation defects may be an important phenomenon and may be significant in case of slowly cooled or reheated basement rocks to test this hypothesis we developed a quantitative model including an explicit treatment of recoil damage annealing and their effect on he diffusion kinetics and calibrate it against literature data our model is based on two hypotheses 1 helium is in equilibrium between an apatite crystal and its defects and 2 alpha recoil damage annealing can be described analogously to fission track annealing this model has been embedded into a monte carlo simulation of helium production ejection diffusion and applied to data from the french massif central a complex slowly cooled terrain with burial reheating where the thermal history has been constrained by previous fission track ft data including ft length distributions u th he ages are close to the ft ages from the same samples and are generally reproducible among replicates but some samples present he age dispersion that is not correlated with crystal size our model reproduces the massif central data very well except for three samples where he ages are older than corresponding ft ages we show that annealing of irradiation damage has an important impact on retentivity of helium and that the he content he is only a rough approximation of the damage level in particular our results show that independence of he ages on crystal sizes in case of reheated samples is a clear indication of the higher he retentivity induced by recoil defects and that an explicit treatment of defect annealing is required for a correct interpretation of u th he ages in such a case more generally a correlation with the crystal size can bring information on the thermal path only if the age of defects well represented by the fission track age is available due to the dependence of the partial retention zone on damages conversely in case of rapid cooling or for samples having low u and th contents damage effects can be ignored without significant effects on he ages

accounting for long alpha particle stopping distances in u th sm he geochronology 3d modeling of diffusion zoning implantation and abrasion in apatite u th he thermochronology the helium distribution in a crystal is a function of the simultaneous processes of radiogenic production thermally activated volume diffusion and the ejection of he caused by long alpha stopping distances these processes are further complicated by zonation of u th and sm within the grain and implantation of 4he from neighboring u th sm bearing minerals we use a refined version of the 3d monte carlo diffusion code of gautheron and tassan got 2010 to simulate the interplay between ejection and diffusion with or without zonation ejection and abrasion for a suite of thermal histories we examine the phenomenon of over correction produced by the alpha ejection correction parameter ft or fzac for homogeneous or heterogeneous eu repartition by comparing the raw measured and ft or fzac corrected ages for a number of scenarios to the ejection free age aef which we define as the age that would be obtained if alpha ejection had not occurred or equivalently if the stopping distance was zero we show that the use of ft or fzac corrected ages generally reproduces the ejection free age to within typical u th he uncertainties 8 even for zoned apatites we then quantify the effect of alpha implantation on u th he ages showing that implantation from a single external source with modest relative u or th enrichment can generate as much as 50 excess he for more extreme cases where an apatite is surrounded by multiple external sources the measured age can be 300 of that determined from an isolated crystal while abrasion of the outer 20 25 m can significantly reduce the age dispersion for rapidly cooled samples slowly cooled samples can still retain 10 30 excess he the removal of the rim of the crystal reduces the thermal information from very low temperatures 40 c and introduces additional technical complications and biases and should therefore be used with caution overall we demonstrate that although zonation and implantation may not be routinely determined we now have the 3d modeling capability to fully investigate and constrain the causes of age dispersion within a sample leading to significant improvement in our ability to interpret u th he data

3d stratigraphic forward numerical modelling approach for prediction of organic rich deposits and their heterogeneities abstract the source rock characteristics e g thickness lateral extension richness are known to be highly variable in both time and space the lower jurassic formations of north western europe contain source rocks with organic rich intervals showing different characteristics from one region to another the paris basin differs from the south east basin of france but organic content and hydrogen index also vary within a single basin during the early jurassic the overall depositional environment of north western europe corresponded to an epicontinental domain at the western extremity of the tethys ocean the early transgressive phase of the jurassic induced flooding of this european realm because of the evolution of the connections and threshold of the european basins and their associated sedimentary settings this domain occupied a key position for the deposition of organic rich layers using a forward modelling approach we aim to predict the heterogeneous characteristics of such sediments it is widely accepted that primary productivity and preservation are key factors favouring the accumulation of organic rich layers however the roles of these factors remain to be assessed and the processes leading to accumulation and preservation need to be quantified we focus on the lower jurassic hettangian to toarcian in france since this provides a vast amount of data to calibrate the 100 kyr time step model in terms of an accurate framework including paleogeography stratigraphy lithology palaeontology mineralogy and geochemistry results show that favourable conditions for the deposition of organic rich sediment can occur even with moderate primary productivity and without requiring anoxic conditions these windows are firstly dependent on internal factors directly linked to the basin morphology such as sedimentation rate bathymetry and distance from the coastline we do not need to evoke external factors such as oceanic anoxic events or enhancement of primary production to account for the deposition of organic rich sediments

successive post variscan stress fields in the french massif central and its borders western european plate comparison with geodynamic data abstract structural analysis and particularly microtectonic methods have made it possible to define the different stages of brittle deformation of the massif central basement and the surrounding sedimentary cover from the end of the hercynian orogeny to the end of the tertiary during the stephanian a compressional tectonic regime prevailed regional faults appeared or were react vated reactivation as a result of initial n s compression becoming nw then e w these regional strike slip faults caused local extension which led to the formation of small coal bearing basins this compressional regime which marked the end of the formation of pangea was followed by a series of extension episodes permian triassic extension oscillating around n s e w to nw se extension in the early and middle jurassic and finally n s to ne sw extension in the late jurassic to cretaceous the normal faults formed during these episodes strongly influenced the distribution of emerging continents and sedimentary basins from the campanian 75 ma to the present the convergence of africa and eurasia involved the distribut on of stresses in the west european plate several tectonic episodes are distinguished in the massif central during the eocene approximately n s compression predominated general e w extension in the late eocene oligocene resulted in grabens with general northerly strike mainly in the centre of the massif central and on its east and southeast borders lastly compression varying from nw se to e w in the north and south of the massif central prevailed during the alpine orogenic phase at the end of the miocene these successive stages of brittle deformation are interpreted in the context of the evolution of the west european plate and its displacement in relation to the african plate the correspondences between the major geodynamic periods and the distribution of stresses over the west european continent are noted as well as the problems which remair to be resolved

sources of crustal deformation associated with the krafla iceland eruption of september 1984 a decade long plate boundary rifting episode in northern iceland ended with the september 1984 fissure eruption of krafla volcano we apply a nonlinear inversion method to geodetic data collected before and after the eruption to infer the location geometry and strengths of deformation sources associated with the eruption the net outflow of magma from a 3 km deep magma chamber beneath the krafla caldera was 30 120 106 m a similar volume of magma 50 70 106 m was emplaced in a 1 meter wide 9 km long dike extending from the surface to 7 km depth furthermore at least 110 106 m of magma erupted accordingly a surplus of magma must have been expelled from a second reservoir the location of which although uncertain is likely to lie at depths greater than 5 km beneath krafla volcano it would be difficult to detect this deeper source because of the narrow aperture of the geodetic networks

where are the limits of mesozoic intracontinental sedimentary basins of southern france abstract reconstitution of the geometry of sedimentary basins is fundamental to understand the nature of present sedimentary rocks and the economic potential in hydrocarbon and mineral resources present day topography of southern france shows elevations growing between the meso cenozoic aquitaine basin to the south east basin across the variscan domain and the jurassic causses small basin with maximum relief in the cevennes area present day elevation offset is of approximately 1000 m this geometry questions the paleogeography and dynamics of these various domains and the relative elevation of the variscan domain during the subsidence of adjacent aquitaine and south east basins in this study we investigate the geological history of the variscan basement high and the causses small basin using paleotemperatures deduced from organic matter analysis low temperature thermochronology and regional geological constraints lower jurassic upper pliensbachian and toarcian marls sampled across the area from the aquitaine basin to the south east basin have similar depositional environments containing mainly type iii oganic matter and close tmax values ranging between 430 and 440 2 c these data show that the entire south massif central has undergone a similar burial history considering that these values are explained by burial low temperature thermochronology data have been acquired on basement rocks outcropping on the borders of sedimentary basins rouergue cevennes and margeride fission track ages are ranging between 74 5 and 187 6 ma and track lengths between 11 5 0 3 and 13 6 0 1 m apatite u th he corrected ages are ranging between 65 5 and 184 15 ma data inversion with the software qtqt indicates a cooling episode starting at the end of early cretaceous or beginning of late cretaceous from maximum temperature of 100 10 c in the rouergue and cevennes area and from 80 10 c in most of the margeride area thermal indicators are compatible with the erosion of a middle upper jurassic and cretaceous sedimentary cover of 1400 400 m assuming a thermal paleogradient of 35 c km the preserved sedimentary cover attests of a middle and upper jurassic lower cretaceous sedimentary cover of 1000 2000m in the aquitaine and south east basins this erosion phase occurred during mid cretaceous and is associated to a major geodynamical event characterized by large amplitude from the aquitaine basin to durancian doming in the south east basin and by kilometric offset we interpret also these data to show that marine connections have existed between the aquitaine basin and the south east basin during the jurassic and likely early cretaceous the present day morphology of the area has then been acquired after cretaceous times and may result from the pyrenean orogenic event during eocene times

variety and relationships of weathering features along the early tertiary palaeosurface in the southwestern french massif central and the nearby aquitaine basin abstract palaeoweathering features and palaeosurfaces in the southwestern french massif central and its margins provide a basis for understanding the evolution of tertiary continental areas recurrent sequences of weathering periods erosion phases and sedimentary deposits have been identified and related to major sea level changes tectonic movements and palaeoclimates the palaeoweathering features are different according to their palaeogeographic position from basin margins to uplands and in limestone platforms or crystalline areas on the basin margins palaeokarst features prevail claystones together with lignite accumulated in these sinkholes later weathering and oxidation of these organic rich deposits led to highly acidic environments resulting in dissolution of the primary clay minerals and formation of secondary clay deposits with gibbsite halloysite montmorillonite nontronite but also of opal ct silcretes and ferricretes palaeoweathering profiles fossilised at the edge of the subsiding basin show red and ochre sands on which red brown silicified duricrust developed later invaded by palygorskite rich calcrete horizons on the limestone borderland behind the subsiding area various palaeokarsts developed extensively surficial palaeokarsts developed after withdrawal of the jurassic sea and buried by cretaceous sandy deposits have been reactivated during the early tertiary later the deep subsurface palaeokarsts from the early tertiary were sealed and then reactivated as they were brought back to the surface by erosion the hinge zone between the basin and the uplands appears relatively stable despite a long history palaeolandscapes here are mostly polygenic with exhumed surfaces related to early mesozoic continental evolution which formed the basis for the development of younger morphologies the tertiary landscapes are characterised by inselbergs of unweathered crystalline rocks rising abruptly above radiating low angle glacis the wide crystalline uplands were early exhumed from their mesozoic cover and underwent deep erosion only scarce marks of a long continental history remain on these upland areas and palaeoweathering features are mainly restricted to grabens the graben deposits show red kaolinitic palaeosols at their base which were later impregnated by silica deposition green smectitic claystones interbedded with calcrete horizons overlay the kaolinitic palaeosols extensive silcretes developed at the top of these deposits despite the wide variety of the palaeosurface compounds there are many similarities the climate change from warm and very wet during the late cretaceous and early tertiary toward warm and dry during the late eocene and early oligocene is the main factor that led to these similarities the oldest testimonies indicate kaolinite rich soils developed on smooth undulated hills the onset of drier climates led to the development of a contrasted mineralogical sequence with inheritance of the kaolinitic materials from the palaeosols development of silicified duricrusts on the piedmonts and glacis and calcretes with mg rich clays in the flood plains and lacustrine environments tectonics also played a major role the early tertiary alpine movements led to the erosion of the old weathering mantles and to reshaping of the landscapes by several erosion cycles drop of the base level led to incision of large valleys on the uplands and development of wide glacis and deep karst network on the margins of the basin on the other hand subsidence or general rise of the base level protected some parts of the landscape from further weathering and erosion the inheritance of pre existing landscapes also is of noteworthy importance the thick kaolinitic mantle inherited from the humid climate has strongly prepared the shaping of the subsequent landscapes the clearing of soft weathering cover exhumed ancient surfaces in the hinge zone between the basin and the uplands whereas in the crystalline uplands it exposed a knobbly etch surface reproducing the ancient topographies present landscapes of aquitaine and southwestern massif central still exhibit many remnants of a long and complex continental history that track back to the early tertiary and beyond

damage formation associated with bending under a constant moment abstract deformation within the earth s lithosphere is largely controlled by the rheology of the rock fracture and faulting are characterized by elastic rheologies with brittle mechanisms while folding and flow are characterized by plastic and or viscous rheologies due to ductile mechanisms however it has been recognized that deformation that resembles ductile behavior can be produced within the confines of the brittle lithosphere specific examples are folds that form in the shallow crust steep hinges at subduction zones that are accompanied by seismicity and large scale deformation at plate boundaries in these cases the brittle lithosphere behaves elastically with fracture and faulting yet produces ductile behavior in this paper we attempt to simulate such ductile behavior in elastic materials using continuum damage mechanics engineers utilize damage mechanics to model the continuum deformation of brittle materials we utilize a modified form of damage mechanics that represents a reduction in frictional strength of preexisting fractures and faults we use this empirical approach to simulate the bending of the lithosphere under the application of a constant moment we use numerical simulations to obtain elastostatic solutions for plate bending and where the longitudinal stress at a particular node exceeds a yield stress we apply damage to reduce young s modulus at the node damage is calculated at each time step by a power law relationship of the ratio of the yield stress to the longitudinal stress and the yield strain to the longitudinal strain this results in the relaxation of the material due to increasing damage to test our method we apply our damage rheology to an infinite plate deforming under a constant bending moment we simulate a wide range of behaviors from slow relaxation to instantaneous failure over timescales that span six orders of magnitude using this method stress relaxation produces elastic perfectly plastic behavior in cases where failure does not occur for cases of failure we observe a rapid increase in damage leading to failure analogous to the acceleration of microcrack formation and acoustic emissions prior to failure the changes in the rate of damage accumulation in failure cases are similar to the changes in b values of acoustic emissions observed in triaxial compression tests of fractured rock and b value changes prior to some large earthquakes thus continuum damage mechanics can simulate the phenomenon of ductile behavior due to brittle mechanisms as well as observations of laboratory experiments and seismicity

late pliensbachian early toarcian early jurassic environmental changes in an epicontinental basin of nw europe causses area central france a micropaleontological and geochemical approach abstract we present an integrated work based on calcareous nannofossil and benthic foraminiferal assemblages and geochemical analyses of two upper pliensbachian lower toarcian sections located in the central south france the studied sections tournadous and saint paul des fonts represent the proximal and the distal part respectively of the jurassic causses basin one of the small partly enclosed basins belonging to the epicontinental shelf of the nw tethys at the transition from late pliensbachian to early toarcian the causses basin recorded an emersion in response to the global sea level fall our data indicate severe environmental conditions of marine waters including salinity decrease and anoxia development occurring in the early toarcian the acme of this deterioration coincides with the early toarcian anoxic event t oae but due to the restricted nature of the basin anoxia persisted until the end of the early toarcian mainly in the deeper parts of the basin the micronutrients and organic organic matter fluxes were probably high during the entire studied time interval as shown by nannofossil and foraminiferal assemblages however nannoplankton production drastically decreased during the t oae as demonstrated by very low nannofossil fluxes and only taxa tolerant to low saline surface waters could thrive at the same time benthic foraminifers temporarily disappeared in response to sea bottom anoxia our study demonstrates that environmental changes related to the t oae are well recorded even in small partly enclosed basins of nw europe like the causses basin within this area the effects of global changes like sea sea level and temperature fluctuations are modulated by local conditions mainly controlled by the morphology of the basin

mantle exhumation crustal denudation and gravity tectonics during cretaceous rifting in the pyrenean realm sw europe insights from the geological setting of the lherzolite bodies 1 the pyrenean peridotites lherzolites form numerous small bodies of subcontinental mantle a few meters to 3 km across exposed within the narrow north pyrenean zone npz of mesozoic sediments paralleling the north pyrenean fault recent studies have shown that mantle exhumation occurred along the future npz during the formation of the albian cenomanian pyrenean basins in relation with detachment tectonics this paper reviews the geological setting of the pyrenean lherzolite bodies and reports new detailed field data from key outcrops in the bearn region only two types of geological settings have to be distinguished among the pyrenean ultramafic bodies in the first type sedimented type or s type the lherzolites occur as clasts of various sizes ranging from millimetric grains to hectometric olistoliths within monogenic or polymictic debris flow deposits of cretaceous age reworking mesozoic sediments in dominant proportions as observed around the lherz body in the second type tectonic type or t type the mantle rocks form hectometric to kilometric slices associated with crustal tectonic lenses both crustal and mantle tectonic lenses are in turn systematically associated with large volumes of strongly deformed triassic rocks and have fault contacts with units of deformed jurassic and lower cretaceous sediments belonging to the cover of the npz these deformed mesozoic formations are not older that the aptian early albian they are unconformably overlain by the albian cenomanian flysch formations and have experienced high temperature low pressure mid cretaceous metamorphism at variable grades such a tectonic setting characterizes most of the lherzolite bodies exposed in the western pyrenees these geological data first provide evidence of detachment tectonics leading to manle exhumation and second emphasize the role of gravity sliding of the mesozoic cover in the preorogenic evolution of the pyrenean realm in the light of such evidence a simple model of basin development can be inferred involving extreme thinning of the crust and mantle uprising along a major detachment fault we demonstrate coeval development of a crust mantle detachment fault and generalized gravitational sliding of the mesozoic cover along low angle faults involving triassic salt deposits as a tectonic sole this model accounts for the basic characteristics of the precollisional rift evolution in the pyrenean realm

the early toarcian jurassic anoxic event stratigraphic sedimentary and geochemical evidence detailed paleogeographic studies in the southern alps suggest that an oxygen minimum model is appropriate for interpreting conditions on tethyan continental margins advection of manganese in this low oxygen layer may explain the local occurrence of carbonate ores of this element carbon sulfur and iron sulfur ratios although showing considerable scatter suggest that bottom waters at this time were locally euxinic containing free hydrogen sulfide this oceanic anoxic event was preceded by significant faunal turnover of ammonites in tethys and accompanied by widespread extinction of benthos in northern europe in response to the lateral spread of anoxic bottom waters during transgression similar changes may be recognized in other parts of the world other anoxic events may have taken place during the jurassic but documentation is as yet meagre furthermore models for such phenomena remain largely speculative although upwelling and increased planktonic productivity commencing in pre toarcian time are favored for the falciferum zone event documented here from author

post rifting stress relaxation at the divergent plate boundary in northeast iceland interaction of the elastic lithosphere with the underlying anelastic asthenosphere causes strain to propagate along the earth s surface in a diffusion like manner following tectonism at plate boundaries this process transfers stress between adjacent tectonic segments and influences the temporal tectonic pattern along a plate boundary observations of such strain transients have been rare and have hitherto been confined to strike slip and underthrusting plate boundaries1 here we report the observation of a strain transient at the divergent spreading plate boundary in iceland a global positioning system survey undertaken a decade after an episode of dyke intrusion accompanying several metres of crustal spreading reveals a spatially varying strain field with the expected diffusion pulse shape and an amplitude three times greater than the 5 7 cm that would be expected from the average spreading rate2 a simple one dimensional model with a thin elastic layer overlying a viscous layer fits the data well and yields a stress diffusivity of 1 1 a 0 3 m2s 1 combined with struc tural information from magnetotelluric measurements this implies a viscosity of 0 3 2 1019 pa s a value comparable to that derived for iceland from post glacial rebound23 but low compared with estimates for mantle viscosity obtained elsewhere3

a precise 40ar 39ar reference age for the durango apatite u th he and fission track dating standard the distinctive durango apatite is widely distributed as a reference material for mineralogy geochemistry and thermochronology the apatite occurs within the cerro de mercado open pit iron mine located in the northern portion of durango city mexico it is largely confined to vugs and open fractures within the lower portion of the ore deposit apatite and iron ore formed in situ and concurrently along with emplacement of small felsic intrusions along the southern margin of the chupaderos caldera complex this activity occurred between eruptions of two major ignimbrites from the caldera complex four single crystal sanidine anorthoclase 40 ar 39 ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31 44 0 18 ma 2 for the apatite itself four additional multi crystal measurements are in general agreement with this age also in agreement are a set of 24 u th sm he ages recently determined directly on the apatite which have a mean of 31 02 1 01 ma 1 with a standard error s e of 0 22 ma

pedogenic and karstic features at the boundaries of bathonian depositional sequences in the grands causses area southern france stratigraphic implications abstract several exposure surfaces d1 to d6 underlain by paleosols paleokarstic surfaces and subsurface paleokarsts were identified in the middle to upper bathonian calcaires a stipites and dolomies ii formations in southern france two kinds of paleosols with different degrees of maturity were recognized simple ferruginous crusts capping very irregular bed surfaces and a rooted horizon the paleokarstic surfaces are marked by nodular horizons and paleocaves on the cevennes shoal the paleokarsts discontinuities are associated with synsedimentary tectonic processes which did not extend into the overlying dolomies ii formation subsurface paleokarsts were observed in the cirque du bout du monde on the cevennes shoal within the calcaires a stipites formation just beneath surface d5 they are characterized by stronger brecciation of the beds most of the paleokarstic discontinuities described in this study correspond to the boundaries of four third order depositional sequences bt 1 to bt 4 d1 at the base of bt 1 d2 d3 d4 and d5 capping bt 1 bt 2 bt 3 and bt 4 respectively d6 at the top of the dolomies ii formation d1 d2 and d3 paleokarsts are geographically limited to the grands causses graben whereas d4 d5 and d6 are present only on the cevennes shoal geographic trends of paleokarsts confirm the transgressive trend of sequences bt 3 and bt 4 and of the overlying dolomies ii formation towards the shoal d6 paleokarstic features were also observed within the uppermost part of the dolomies ii formation in the horst de saint bresson the latter transgressive process provides evidence for subaerial exposure of this paleostructure during the latest bathonian callovian interval induced by tectonic uplifts

postrifting anelastic deformation around the spreading plate boundary north iceland 1 modeling of the 1987 1992 deformation field using a viscoelastic earth structure a third global positioning system gps survey of a regional network surrounding the krafla volcanic system north iceland was conducted in 1992 following a major crustal spreading episode which began in this system in 1975 differencing the 1992 results with those from 1987 and 1990 reveals a regional deformation field with a maximum rift normal expansion rate of 4 5 cm year near the rift decreasing to 3 cm year at large distances the time averaged spreading rate in north iceland 1 8 cm year cannot account for this deformation the vertical deformation field reveals regional uplift throughout the network area at its maximum closest to the rift and decreasing with distance three different models are applied to study the postdike injection ground deformation 1 stress redistribution in an elastic layer over a viscoelastic half space 2 stress redistribution in an elastic viscous layered medium and 3 continued opening at depth on the dike plane in an elastic half space using model 1 the effects of historical episodes in the region are subtracted from the observed displacement fields and the remaining motion is modeled as relaxation following the recent krafla rifting episode the best fit model involves a half space viscosity of 1 1 x 10 18 pa s a relaxation time of 1 7 years and an elastic layer thickness for northeast iceland of 10 km the vertical field indicates that the krafla dike complex rifted the entire elastic layer using model 2 the motion 1987 1990 and 1990 1992 can be simulated adequately given the survey errors but the 1987 1992 deformation is poorly fitted suggesting that a more realistic geophysical model is required using model 3 a range of dikes will fit the deformation field