an inverted double seismic zone in chile evidence of phase transformation in the subducted slab data from two microseismic field experiments in northern chile revealed an elongated cluster of earthquakes in the subducted nazca plate at a depth of about 100 kilometers in which down dip tensional events were consistently shallower than a family of compressional earthquakes this double seismic zone shows a distribution of stresses of opposite polarity relative to that observed in other double seismic zones in the world the distribution of stresses in northern chile supports the notion that at depths of between 90 to 150 kilometers the basalt to eclogite transformation of the subducting oceanic crust induces tensional deformation in the upper part of the subducted slab and compressional deformation in the underlying mantle

the stratigraphic distribution of ni rich spinels in cretaceous tertiary boundary rocks at el kef tunisia caravaca spain and hole 761c leg 122 abstract ni rich spinels have been found throughhout the world at the cretaceous tertiary boundary these minerals have no counterparts in terrestrial rocks but have been observed in meteoroid ablation material and impact debris suggesting that the cretaceous tertiary boundary spinels result from an extraterrestrial event their stratigraphic distribution which has been observed at the cretaceous tertiary boundary sites of el kef tunisia caravaca spain and hole 761c odp leg 122 indian ocean are markedly different from site to site and are all much less extended than the ir anomalies regional differences in ni rich spinel distibution are entirely explained by a brief event recorded under different bioturbation conditions the wider stratigraphic dispersion of ir is attributed to postdepositional chemical processes at el kef the site which appears to be the least affected by bioturbation and which has the highest sedimentation rate the 1 2 mm thick ni rich spinel spike indicates that the deposition time did not exceed an upper limit of 100 years this upper limit is consistent only with the collision of an asteroid or a comet at the close of the cretaceous

rejuvenation of continental lithosphere beneath an intracratonic basin abstract the effective elastic thickness of the lithosphere in the williston basin region has been determined for several time intervals by fitting an elastic flexure equation to the shape of the basin over time the elastic thickness increases from about 40 km just after the basin began subsiding 450 m y b p to about 80 km at the present these results agree well with the increasing elastic thickness with age predicted for a cooling 250 km thick plate taking elastic thickness to be depth to the 450 c isotherm if we assume a thermal age of the lithosphere of about 500 m y radiometric dates of basement in the area however generally yield ages of 1 7 b y churchill province and 2 5 b y superior province because basement ages are generally much greater than the inferred lithospheric thermal age in the williston basin region we suggest that lithosphere in this area was thermally rejuvenated about 500 m y b p by the same process es responsible for the thermal subsidence of the basin temperatures were not high enough in the upper crust to reset most radiometric clocks additional support for a thermal event 500 m y b p comes from fission track ages and a single rb sr age of basement rocks of about 500 m y lithospheric rejuvenation has been modelled using a one dimensional finite difference thermal model the base of a 250 km thick plate is heated from 1333 c to 1850 c approximately the solidus of basalt at that depth for approximately 100 m y with conduction only the depth to the 450 c isotherm slowly decreases then slowly increases after heating ceases rapid thinning and slow thickening of the lithosphere can be achieved only if convective thinning is simulated in the model the model with convection yields results consistent with the observed increase in lithospheric thickness as well as the observed subsidence of the basin over time

mechanisms of uplift preceding rifting abstract mareschal j c 1983 mechanisms of uplift preceding rifting in p morgan and b h baker editors processes of continental rifting tectonophysics 94 51 66 the feasibility of three mechanisms that have been proposed to account for the uplifts and heat flow anomalies which might precede rifting is examined the conductive heating following an increase in the heat flux at the base of the lithosphere could result in large scale uplifts 1000 m for 15 mw m2 change in heat flux on a time scale of 108 years a pulse of additional heat flow could induce a more rapid uplift but not the surface heat flow anomaly which is always delayed the advection of heat by magma intrusion into the lithosphere could also produce the uplift and heat flow anomaly if magmas are injected at a steady rate an uplift of the order of 1000 m can be induced in about 50 106 years but it requires the rate of magma injection to be equivalent to 0 3 of the total volume of the lithosphere per 106 years a short intense episode of magma invasion could explain a rapid uplift and change in surface heat flow but it requires the replacement of about 30 of the total volume of the lithosphere the complete replacement of the lithosphere by the rise of a diapir of lighter and hotter asthenospheric material has been modelled numerically the model indicates that the rise of the asthenospheric diapir can take place in a very short time 3 106 years and produce the surface uplift and the crustal thinning observed in rift systems provided that following heating the lower lithosphere viscosity is reduced to 1021 pa s

dynamic link between the level of ductile crustal flow and style of normal faulting of brittle crust abstract in a rheologically layered crust compositional layers have an upper elasto plastic part and a lower viscous one when broken the upper elastic part undergoes flexure which is upward for the foot wall and downward for the hanging wall as a consequence of bending stresses will develop locally that can overcome the strength of the plate and therefore impose the migration of active fault in the lower viscous part of each compositional layer rocks can potentially flow numerical modelling of the behaviour of a crust made up of two compositional layers during and following extension shows that flow can take place not only in the lower crust but also and more importantly in the lower part of the upper crust the ability of crustal rocks to flow influences the style and kinematics of rifted regions when no flow occurs subsidence will affect the extending areas both hanging wall and foot wall will subside with respect to an absolute reference frame such as sea level and there will be a strict proportionality between extension and thinning in addition the downward movement of the fault blocks will decrease the local stresses created in the foot wall and increase those of the hanging wall thereby imposing a migration of fault towards the hanging wall this is the behaviour of extensional settings developed on stabilised crust and which evolved in a passive margin when flow does take place middle crustal rocks will move towards the rifting zone causing isostatically driven upward movements that will be superimposed on movements associated with crustal and lithospheric thinning consequently fault blocks will move upwards and the crust will show more extension than thinning the upward movements will decrease the stresses developed in the hanging walls and increase those of the foot wall faults will then migrate towards the foot wall such a mode of deformation is expected in regions with thickened crust and has its most apparent expression in core complexes

role of crustal stretching on subsidence of the continental crust abstract the principal areas of subsidence of the continental crust are wide intracontinental basins and passive continental margins in many intracontinental basins crustal extension amounts to only a few percent and is unable to explain their subsidence as shown for the timan pechora pre caspian and west texas basins the deep black sea and south caspian basins are underlain by highly attenuated continental crust having velocities in the 7 km s range that shows no evidence for significant extension in passive margins upper crustal extension ranges between 10 and 40 whereas the lower crust is attenuated by 2 3 times e g campos basin north biscay margin large discrepancy between upper crustal stretching and lower crustal attenuation is seen in the crustal configuration of many intracontinental grabens e g baikal and dniepr prypyat basins only in highly extended areas such as the basin and range province and the east china rift system does crustal stretching account for the observed crustal thinning the observed discrepancy between upper extension and crustal thinning across intracontinental rifts and passive margins and particularly in some wide intracontinental basins suggests that crustal stretching alone cannot account for the subsidence of these basins phase transformations in the lower crust probably facilitated by the presence of hydrous fluids can cause rapid 1 ma as well as long term 200 1500 ma upward displacement of the moho discontinuity resulting in high amplitude subsidence of the crust in areas of crustal extension this mechanism can overprint subsidence induced by thermal relaxation of the lithosphere the case of the black sea and south caspian basins shows that density increase of the lithosphere can also occur independently of crustal stretching

preliminary evidence for the involvement of budding bacteria in the origin of alaskan placer gold lacelike networks of micrometre size filiform gold associated with alaskan placer gold particles are interpreted as low temperature pseudomorphs of a pedomicrobium like budding bacterium submicron reproductive structures hyphae and other morphological features similar to those of pedomicrobium manganicum occur as detailed three dimensional facsimiles in high purity gold in and on placer gold particles from lillian creek alaska in a scanning electron microscope survey the majority of gold particles at nine alaskan placer deposits appear to include gold that has accumulated chemically at low temperatures in and on the cells of p manganicum similar bacterioform gold from a paleozoic deposit in china and from the precambrian witwatersrand deposit in south africa may indicate that bacterioform gold is widespread

chronology of a sedimentary sequence from the land ocean interaction zone in the north jiangsu basin abstract the north jiangsu basin njb located in the western part of the north jiangsu south yellow sea basin is a critical region providing invaluable archives about the evolution processes of the land ocean interchange and climatic environments in a coastal zone chronology is of crucial importance to reconstruct the timing of sea level eustacy and shoreline changes as well as the evolution of the regional environment by means of numerical dating to determine the age of sedimentary strata in the sea land interaction zone of the njb however sediment dating in the njb remains controversial due to limitations in the quantity and quality of chronological data therefore the sediments in the jianhu gangxi gx profile of the njb were analyzed in this study by applying both fine grained quartz optically stimulated luminescence osl dating and 14c dating the results were used to discuss the processes of regional environment evolution the osl dating of fine grained quartz components in sediments was found to be a reliable method to determine deposition ages of sediments in the land sea interaction zone in the njb the combination of osl and accelerator mass spectrometry 14c dating techniques allowed a reliable and high resolution chronological framework for the gx profile since the last glaciation the sedimentary environment in the eastern portion of the njb changed significantly in particular 1 between 13 and 7 25 ka b p the area of interest was dominated by lacustrine deposition within a stable sedimentary environment with low deposition rates 2 from 7 25 to 6 45 ka b p marine transgression events occurred and the deposition rate increased rapidly in an area of land sea interaction 3 after 6 45 ka b p a regression event occurred and the study area transformed into a lowland lake environment

iridium emissions from kilauea volcano during may 1983 gas and particulate samples were collected at the cooling vents of kilauea volcano in hawaii three vents on the southeast rift zone were sampled using base treated filter packs to absorb acidic gases and teflon filters for particles the samples were analyzed for 40 elements by nondestructive neutron activation analysis as with other volcanoes the chalcophilic and volatile elements were enriched by up to seven orders of magnitude relative to the erupted basalt unlike the case of other volcanoes iridium was observed to be highly enriched 105 fold at the two higher temperature vents and the degree of enrichment appears to be related to both high temperatures and high fluorine content of the gases estimates of the emission rates of ir during eruptions of kilauea based on the measurements reported here and those of the hawaiian volcano observatory yield an emission rate of about 3 g ir per 106 m3 of magma this rate amounts to only about 0 3 of the ir present in the magma that is being released by the volcano this previously unidentified ir source to the atmosphere and ocean may have a significant influence on the geochemical cycle of ir on the earth the uniqueness of kilauea in its emissions of ir is probably related to the deep source of magma which contains significant levels of ir 0 32 ppb and high levels of the halogens f and cl

numerical modeling on the relationship between thermal uplift and subsequent rapid subsidence discussions on the evolution of the tainan basin existing evidence shows that an oligocene erosion event occurred on the northern continental margin of the south china sea and the tainan basin area might be at the center of this event followed by a rapid tectonic subsidence in the late oligocene and early miocene period the rapid tectonic subsidence is mainly thermal controlled and the effect of the yichu fault on the tainan basin is limited to the basin s eastern part we develop a 2 d thermal mechanical kinematic numerical model to explore the relationship between thermal uplift and subsequent rapid subsidence in the tainan basin our modeling indicates that the oligocene uplift erosion and subsequent rapid subsidence could be caused by a thermal event and the differential subsidence of the basement caused by thermal contraction can initiate the development of small faults however it also suggests that other mechanisms might be needed to jointly account for the observed erosions citation shi x h xu x qiu k xia x yang and y li 2008 numerical modeling on the relationship between thermal uplift and subsequent rapid subsidence discussions on the evolution of the tainan basin tectonics 27 tc6003 doi 10 1029 2007tc002163

phase change and thermal subsidence of the williston basin summary a mechanism of subsidence for the williston basin is proposed and tested the mechanism combines cooling and thermal contraction of the lithosphere with a phase transformation in the lower crust the cooling and the phase boundary movement following a sudden change in the thermal boundary condition at the base of the lithosphere are calculated isostatic adjustments and sea level variations are included to determine subsidence and sediment accumulation at the basin s centre which are compared with the data the subsidence record could not be explained by temperature boundary conditions the calculations show that 1 the heat flow boundary condition explains the long duration more than 350 myrs of subsidence of the williston basin and 2 the delay of the phase change subsidence about 40myrs explains an acceleration of subsidence in the early stages of the basin s evolution when isostatic adjustments and the effect of sea level variations are included the calculated sediment accumulation history fits well the sediment record of the williston basin

the effect of lithospheric phase transitions on subsidence of extending continental lithosphere abstract a new simple model for sedimentary basin formation which combines both stretching and phase transitions occurring together in the lithosphere is proposed and two important aspects in sedimentary basin formation are addressed 1 the explanation of the domal uplift preceding rifting and 2 the ability of the model to explain the contradiction between a small amount of crustal stretching and the large amount of thermal subsidence without the need to invoke differential stretching between the crust and mantle relationships for synrift initial and thermal post rift subsidence are derived the model predicts that extension of continental lithosphere could produce synrifting uplift up to a threshold amount of extension and then subsidence for greater extension three dimensionless parameters quantify the contribution of the phase transition to the subsidence history 1 the clapeyron slope normalized to the initial geothermal gradient 2 the ratio of depth of phase transition to the thickness of the lithosphere and 3 the ratio of density effect due to phase transition to the change in density due to thermal expansion an evaluation of these parameters has been carried out by using thermodynamical data for the garnet spinel transition for upper mantle rocks our simple model shows that by including the effects of lithospheric phase transition in existing stretching models of sedimentary basin formation we can obtain an extra significant parameter controlling the subsidence and the entire extensional tectonics of such basins

from high plateaux to marginal basins the active role of the mantle current geodynamic models pose many serious problems for the description of continental tectonic processes serious difficulties arise also in the description of some of the ocean basins deformation of the earth s crust appears to be concentrated along several mobile megashear belts it is proposed that these could be the surficial expression of mantle chimneys which extend to great depths within these structures post orogenic high plateaux form and as time passes these develop a central depression enclosed within arcs or indeed full scale rings of mountains the enlargement and deepening over time of the back arc depression occurs in concert with the radial migration of a topographic wave in this respect the following structures represent different phases in an evolutionary sequence a the tibetan plateau b the north american cordillera and c the tyrrhenian basin and calabrian arc the development of this succession reflects a particularly high degree of activity in the mantle in which diapiric rising of asthenospheric matter elevates the land and causes tensional rifting the resulting centrifugal movement of the mantle melt results in crustal foundering and engulfment of the area behind the arc which thus becomes susceptible to the subsequent processes of com pressional deformation

magnetostratigraphy late devonian iridium anomaly and impact hypotheses paleomagnetism sedimentology and fine scale stratigraphy have been integrated to explain the origin of an iridium anomaly in the late devonian of western australia thermal demagnetization experiments were carried out on 93 specimens of marginal slope limestone form the northern canning basin samples are from a condensed sequence of deep water 100 m frutexites microstromatolites frutexites is a shrublike cyanobacterial organism that probably precipitated hematite or a metastable precursor from sea water when plotted within the microstratigraphic framework for the study area the observed characteristic directions from the sampled interval 14 5 cm thick are in five discrete layer parallel normal and reversed polarity zones the measured northeast southwest declinations and shallow inclinations probably record late devonian magnetostratigraphy on a centimetre scale the frutexites bed studied there occurs close to the frasnian famennian late devonian boundary a time of mass extinction of a wide variety of marine organisms throughout the world anomalously high iridium concentrations observed in the frutexites bed have suggested to some authors that the mass extinction was caused by meteorite impact this study concludes that iridium which is present over the span of five layer parallel magnetic reversals was concentrated over a long period of time by biologic processes thus the canning basin iridium anomalymore may be unrelated to meteorite impact less

post depositional mobility of platinum iridium and rhenium in marine sediments the discovery of high concentrations of iridium in cretaceous tertiary boundary sediments engendered the hypothesis1 that a meteorite collided with the earth 65 million years ago coincident with the mass extinction that occurred at that time iridium spikes of various magnitudes have subsequently been reported at more than 10 other extinction horizons2 11 it has been suggested on the other hand that geochemical processes might create or modify many of these spikes5 11 4 but a critical evaluation of these suggestions has been hindered by incomplete understanding of low temperature iridium geochemistry other platinum group elements ru rh pd re os pt au are often found to be associated with ir spikes and inter element ratios have been used to assess the cosmic or terrestrial nature of the enrichments5 7 9 28 29 but the geochemical influences on these relative abundances are also poorly constrained here we describe analyses of recent abyssal marine sediments which allow us to characterize the behaviour of pt re and ir during early diagenesis these elements are redistributed by changes in sedimentary redox conditions such changes can probably account for many of the small platinum group element spikes found in the geological record and may render ambiguous attempts to interpret inter element ratios

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

stretching factor estimation for the long duration and multi stage continental extensional tectonics application to the baiyun sag in the northern margin of the south china sea abstract conventional subsidence analysis methods are generally based on the assumption of instantaneous stretching which totally ignores the heat loss during continental rifting for long duration or multi episodic extensional tectonics such assumption is no longer valid and could cause significant deviation from real subsidence here i extend original finite stretching model to a multi stage scenario in terms of geothermal and structural successions between two adjacent rifting events a procedure is designed to estimate stretching factor by fitting the whole tectonic subsidence history in a self consistent manner on the basis of the extended model i apply this procedure to the baiyun sag in the continent slope of the south china sea subsidence analysis shows that the sag is characterized by two episodes of rifting the first stage 49 to 23 8 ma resulted in stretching factors in the range of 1 3 to 4 0 with the greatest stretching at the sag center while the second one 17 5 to 10 5 ma yielded minor stretching factors of 1 1 to 1 2 these estimates agree well with that determined from crustal thinning for the northern and central potions of the sag but significant discrepancy occurs between the southern edge of the sag and the present continent ocean boundary where the crustal thickness is less than 20 km and potentially extensive magmatic intrusion was well imaged in the seismic profiles i suggest that the magma assisted rheologically weakening and possible lower crust ductile flow contributed to the extension discrepancy the margin north of the baiyun sag having a relatively thick crust and vacant of magmatic activity was basically stretched in a depth uniform style it is inferred that preexisting weakness in the lithosphere and abundant sediment supply were responsible for the extremely thick cenozoic sediment developed in the baiyun sag

active diagenetic formation of metal rich layers in n e atlantic sediments sediment cores from the porcupine abyssal plain exhibit an indurated layer 0 5 3 cm thick at depths of approximately 50 cm this is some 15 20 cm below the glacial holocene transition as interpreted by radiocarbon dating and the palaeontological criteria of ruddiman and mcintyre 1981 the layer is forming currently at the oxic post oxic boundary in the sediments as revealed by pore water data o2 and no 3 are present in solution above the layer while fe2 mn2 po3 4 and nh 4 are present in solution below and all these species show concentration gradients indicating fluxes into the layer these data are consistent with the hypothesis for the initiation and sustained formation of such layers proposed by wilsonet al 1986a b the elements mn ni co fe p v cu zn and u are all enriched to varying degrees in the vicinity of the layer some differential stratification of these elements in the vertical consistent with a redox control is observed at one site with a 0 5 cm layer with mn ni and co above fe p v and cu in the layer and u below at another site the metal rich layer has higher fe and p concentrations and is more indurated here all enrichments except co are contained within a single layer sample 3 cm thick

physical properties of crustal and upper mantle rocks with regards to lithosphere dynamics and high pressure mineralogy this paper focusses on the physical properties of rocks and property changes occurring during geodynamic processes it addresses a number of outstanding petrophysical problems in the framework of crustal and mantle deformation the major themes to be discussed will be the seismic signature of crustal and mantle shear zones the nature of crustal and mantle anisotropy and the contribution of anisotropic rocks to seismic reflectivity in particular seismic shear waves traversing anisotropic strata can reveal important information on the spatial orientation of deformed structures because the measured seismic anisotropy has a structural origin emphasis will be placed on the relations between shear wave splitting and fabric anisotropy fabric anisotropy and strain and strain and geologic tectonic processes the second part of this paper discusses the effect that metamorphic phase transitions in crustal and mantle rocks may have on rock physical properties velocity density specifically the possible role that the gabbro eclogite transition could have in crustal delamination and recycling of material into the mantle will be considered

tectonics and global change inferences from the late cenozoic subsidence and uplift patterns in the atlantic mediterranean region summary quantitative subsidence analysis for a number of rifted basins in the northern atlantic mediterranean region provides evidence for rapid phases of plioquaternary subsidence the observed acceleration in tectonic subsidence occurs after a phase of general quiescence in subsidence in these basins and deviates from predictions of stretching models the latter indicate a decay of subsidence with time after mesozoic tertiary basin formatioh and a slow tectonic subsidence in plio quaternary times a possible explanation for the observed patterns of anomalous subsidence could be an increase in the level of intraplate compression in the northern atlantic region intraplate stress changes in the plio quaternary are related to the dynamics of african eurasian collision processes and a reorganization of spreading directions in the atlantic possibly reflecting a plate reorganization of global nature it seems that the plio quaternary record reflects a period of increased levels of neotectonic activity interplaying with periods of de glaciation stress induced topography in the onshore parts of continental margins coupled with the stress induced subsidence in the offshore deeper parts of the basins could have contributed to recent phases of uplift in fennoscandia augmenting the uplift induced by glacial unloading estimates of ice thicknesses are directly inferred from the observed uplift ignoring other driving mechanisms whereas topography plays a crucial role in the dynamics of glaciation it is therefore important to quantdy the interplay of rapid tectonic uplift and subsidence phases with climatic effects during the plio quaternary

mineralogy and phase chemistry of an ir enriched pre k t layer from the lattengebirge bavarian alps and significance for the ktb problem abstract three distinct iridium anomalies are exposed in the cretaceous tertiary boundary ktb section of the lattengebirge bavarian alps comprising a pelagic calcareous sequence from below a continental slope multiple ir anomalies are in contrast to most reported k t sections which are characterized by a single ir peak two of these ir spikes are contained in the 20 cm thick actual ktb termed clayey interval with the main peak of 4 4 ppb precisely at the boundary and the second peak of 2 8 ppb 10 cm above a third peak of 3 6 ppb was detected at 16 cm below the ktb in a thin turbiditic and coal bearing sandstone layer of upper maastrichtian age the distribution of ir anomalies provides evidence of episodic ir delivering events over an extended period of time whereas the worldwide distributed single spiked ktb layers are composed entirely of diagenetically altered materials various phases are discernible in the well preserved ir bearing pre k t layer 1 sandstone clastic minerals in a carbonate matrix 2 coal which is partly burned 3 sulfides chalcopyrite included in coal and 4 silicate melt glasses the glasses represent the first ones reported from a k t section and were formed by combustion metamorphism from sedimentary parent rocks of the coal as indicated by their major element chemistry and ree patterns phase chemistry of the layer components reveals ir concentrations of up to 110 ppb indicating that ir and chalcophile trace elements tre are primarily contained in the sulfides this carrier phase points to ir enrichment by processes unrelated to impact and its stratigraphic position demonstrates availability of ir for incorporation into sediments before the ktb the uptake of ir and chalcophile tre into the coal and subsequent combustion metamorphism occurred on the continent during a time interval of 14 000 9000 yr prior to the ktb as constrained by radiometric age determinations stratigraphy and sedimentation rates in the lattengebirge k t sediments no indications of shock are found the results suggest a volcanic origin for the multiple ir bearing horizons like the worldwide ktb layers they are concomitantly enriched in ir and chalcophile tre and hence have a common source the lattengebirge k t profile is largely continuous and was recently reexamined by micropaleontological methods contribution by d herm and k f weidich the preservation of multiple ir anomalies is attributed to high sedimentation rates of 1 75 cm kyr and the absence of carbonate dissolution and condensation of sediments usually encountered in the worldwide ktb layers

terminal cretaceous environmental events the geologic record of terminal cretaceous environmental events indicates that iridium and other associated elements were not deposited instantaneously but during a time interval spanning some 10 000 to 100 000 years the available geologic evidence favors a mantle rather than meteoritic origin for these elements these results are in accord with the scenario of a series of intense eruptive volcanic events occurring during a relatively short geologic time interval and not with the scenario of a single large asteroid impact event

cretaceous tertiary boundary of dsdp site 596 south pacific sediments from the cretaceous tertiary k t boundary at deep sea drilling project site 596 record accumulation of pelagic clays at about lat 45 s in the center of the ancestral south pacific the k t boundary in this region is characterized by oxidized thoroughly bioturbated sediments containing large amounts of ir and relict high pressure and high temperature mineral phases the net ir fluence 320 ng cm 2 is among the largest on earth and the number of shocked quartz grains 30 m in size 1800 cm 2 exceeds that in all localities outside of north america

chemostratigraphy across the cretaceous tertiary boundary and a critical assessment of the iridium anomaly the elevated concentration of iridium one of the platinum group elements pge at the cretaceous tertiary boundary is still the most generally accepted evidence that a large bolide struck the earth at the time of the end cretaceous mass extinctions new chemostratigraphic data for cross boundary sections from both hemispheres are not easily explained in terms of such an impact event for example the observation that the pge patterns show marked differences between the hemispheres the new constraints indicate that models of mantle derived pge should be seriously considered and that pge anomalies might not be as useful as previously thought as unambiguous identifiers of large impact events in the earth s history

a mechanism for crustal thinning without lateral extension in some continental regions thin crust is found without observing significant indications of extensional tectonics we argue that in such regions upwelling mantle flows may give rise to sublithospheric loading stresses which may be able to squeeze out the ductile lower crust but leave the upper crust and sub crustal lithosphere essentially unstretched this model allows us to constrain the viscosity of the lower crust if the amount and time scale of crustal thinning is observed in any subsidence basin we applied our model to the pannonian basin hungary and found lower crustal viscosities between 1020 and 1021 pa s comparing this viscosity with laboratory data of crustal rocks results in lower crustal temperatures of about 600 to 800 c which are in good agreement with geothermal models our model might also help to understand the appearance of a very thick crust and lithosphere without any pronounced topography as it is found in fennoscandia

the gabbro eclogite phase transition and the elevation of mountain belts on venus maxwell montes standing up to 7 km above the adjacent highland plateaus constitute the highest mountain belt on venus because the thickness of the crust is likely to be limited by the gabbro garnet granulite eclogite phase transitions this relief is difficult to reconcile with the assumption of thermodynamic equilibrium and a standard airy isostatic model we explore the hypothesis that the crust mantle boundary is not in phase equilibrium but rather is rate limited by the temperature dependent volume diffusion of the slowest ionic species under the simplifying assumption that the mountains formed by uniform horizontal shortening of the crust and lithospheric mantle at a constant rate we solve the one dimensional thermal evolution problem the time dependent density structure and surface elevation are calculated by assuming a temperature dependent reaction rate and local airy isostatic compensation for a rate of horizontal strain of 10 15 s 1 or greater the rise in temperature at the base of the crust during mountain formation is modest to negligible the deepening lower crust is metastable and surface elevation increases as the crust is thickened for strain rates less than 10 16 s 1 in contrast crustal temperature increases with time because of internal heat production and the lower crust is more readily transformed to the dense eclogite assemblage for such models a maximum elevation is reached during crustal shortening while this maximum relief is 7 km or more for some models a smaller density contrast between crust and mantle than assumed here 500 kg m 3 and incorporation of horizontal heat transport would lessen this value we therefore favor formation of the mountain belt at a strain rate at least of order 10 15 s 1 by this reasoning maxwell montes must be comparatively young of order 50 ma

two dimensional inverse modeling of sedimentary basin subsidence recently developed mathematical models of subsidence in extensional basins provide a good basis for forward modeling however the large number of parameters to be specified by the user makes it difficult not only to do the modeling itself but also to judge the meaning of the results we present a new method for automatic searching of the best fitting parameter set of a 2d basin formation model weighted goal functions are used in the minimization process by the inverse problem solver algorithm the nominal parameter set in the present case includes profiles of the crustal and subcrustal thinning factors and the level of lithosphere necking the method was tested on synthetic data with parameters that were perturbed beforehand with some restrictions the algorithms are capable of resolving such perturbations the inversion technique has been applied to two profiles crossing the dnieper donets basin ukraine the crustal thinning factors obtained argue for a scissors like style of basin opening high subcrustal thinning values are necessary in order to explain the abnormally thick carboniferous section in the basin these values should be treated as cumulative ones due to the currently unresolvable influence of other rifting related processes particularly phase transitions

accretion rate of extraterrestrial particles determined from osmium isotope systematics of pacific pelagic clay and manganese nodules abstract pelagic clay and mn nodules from domes sites in the north pacific and a varved glacial lake deposit from connecticut were analysed for os concentration and isotopic composition by isotope dilution secondary ion mass spectrometry after treatment by nis fusion or oxalic acid leaching bulk pelagic clay from domes site c has 187 os 186 os 6 5 and os 0 14 ng g oxalic acid leaches of this same sediment and of mn nodules from domes sites a and c have more radiogenic 187 os 186 os ratios which average 8 3 bulk glacial lake hitchcock sediment has 187 os 186 os 12 5 and os 0 06 ng g the total os flux to north pacific pelagic clay is 7 to 10 ng os cm2 106 y lake hitchcock sediment provides an integrated value for the local crustal 187 os 186 os ratio the oxalic acid leaches are assumed to attack hydrogenous phases selectively a simple model in which the only sources of os to the ocean are continental crust with the isotopic composition of lake hitchcock 187 os 186 os 12 5 and extraterrestrial particles with 187 os 186 os 1 1 results in a cosmic flux of osmium to the sediment of 4 9 ngos cm2 106 y of which 20 is hydrogenous a model in which the sources of os to the ocean are continental crust with an 187os 186os ratio of 30 from the model of palmer and turekian 1986 oceanic mantle or crust with 187 os 186 os 1 04 and extraterrestrial particles with 187 os 186 os 1 1 results in a cosmic flux of os to the sediment of 5 7 ngos cm2 106 y of which none is hydrogenous these extraterrestrial os fluxes correspond to maximum c 1 chondrite accretion rates of between 4 9 104 and 5 6 104 tonnes y

a phase of transient subsidence sediment bypass and deposition of regressive transgressive cycles during the breakup of iberia and newfoundland abstract seismic outcrop and well data from west iberia and newfoundland are used to investigate sediment stacking patterns during continental breakup as a function of tectonic subsidence in west iberia two breakup sequences are revealed on seismic data by marked strata offlap oceanwards from the present day continental shelf this character is similar to newfoundland where correlative strata comprise lower cretaceous cenomanian coarse grained siliciclastics accumulated around local sediment source areas the interpreted data reveal that the two breakup sequences 1 materialise sediment bypass onto continental slope depocentres that experienced important tectonic subsidence during continental breakup but without showing typical syn rift growth packages 2 generate specific forced regressive stratigraphic intervals that relate to uplift and exhumation of the proximal margin subsidence and sediment stacking patterns in both west iberia and newfoundland reflect similar continental breakup processes as they evolved from the upper lithosphere to their mantle breakup stages on both margins coarse grained siliciclastic units on the proximal margin give rise to thick shaley successions in deep water basins this work also confirms that in a setting dominated by a significant sediment influx yet lacking the burial rates of continental slope basins in newfoundland west iberia comprised accommodation driven basins during continental breakup not necessarily sediment starved as a corollary of our analysis we classify breakup sequences around the world based on the characteristic lithologies of their regressive transgressive depositional cycles

mechanisms to explain the platinum concentration in ferromanganese seamount crusts abstract ferromanganese seamount crusts cover large areas of hard substrates of sea floor edifices in the central pacific they can be distinctly enriched in pt and show pt pd ratios which are several orders of magnitude greater than those orders of magnitude greater than those of seawater thirty four selected samples of the younger and the older crust generation were analyzed for mn fe ni co and pt the bulk pt concentration varies between 0 14 and 1 02 ppm and averages to 0 51 0 24 ppm the older generation is generally somewhat richer in pt 0 63 0 25 ppm than the younger one 0 39 0 15 ppm compared to the earth s upper continental crust the ferromanganese material contains about 100 times more pt in the studied depth range between 1100 and 3800 m the pt concentration is controlled by the water depth in addition to the bulk data the distribution of pt and pd is recorded through a complete ferromanganese crust profile the average pd value amounts to 16 2 8 ppb for the younger and 16 4 8 ppb for the older generation two immediate sources may be responsible for the pt concentration in the ferromanganese crusts seawater and cosmic spherules the presented thermodynamic approaches are based on the presumption that the pt tetrachlorocomplex is the prevailing species under seawater conditions precipitation of pt in seawater may take place if this complex is decomposed by reduction of pt 2 to the elemental state on the other hand the mno 2 precipitation is caused by oxidation of dissolved mn 2 ions which are supplied from the oxygen minimum zone a model reaction by combining the two half reactions is formulated assuming a coprecipitation of pt with mno 2 an alternative mechanism to explain pt enrichment in ferromanganese crusts might be surface adsorption of anionic pt tetrachloro complexes onto positively charged amorphous iron hydroxide particles cosmic spherules were discovered in samples of the older ferromanaganese crust generation the spherules are mostly concentrated in the upper part of this crust layer iron meteorites often have pt concentrations up to 20 ppm the relative quantity recovered in the grain size class 0 1 0 4 mm only amounts to 2 10 4 which is significantly too low to explain the pt content of the crust samples however smaller cosmic particles may exist and also contribute to the pt enrichment suggesting that rough crust surfaces are excellent collectors of denser detrital material the maximum pt values observed in the youngest part of the older crust generation may be explained by physical incorporation of cosmic particles

sedimentology and ichnology of the cretaceous tertiary boundary in denmark implications for the causes of the terminal cretaceous extinction abstract trace fossils and other sedimentologic evidence of bioturbation in latest cretaceous and earliest tertiary marine deposits in denmark provide important clues to understanding the ecology of the sea bottom immediately before during and after the terminal cretaceous boundary event trace fossil associations at all the onshore danish boundary sites indicate a general upward shoaling of the late maastrichtian chalk sea which was accompanied by an abrupt shift from a carbonate ooze sea floor to a marly clay rich unstable substrate at the boundary although the substrate condition may have become more stable in the early danian water depths never did return to their former deep level there is evidence of localized anoxia during the deposition of some boundary sediments in the fish clay in shallow parts of the danish maastrichtian sea but no clear evidence of anaerobic conditions occurring at the era boundary was observed in boundary strata deposited in deeper water near the center of the basin in fact earliest danian marl there is totally bioturbated a pulse of calcite dissolution in shallow water coincided precisely with the era boundary and this event played a major role in the formation of the fish clay in eastern denmark which is a condensed series of smectitic clay rich layers from which much calcite has dissolved the effects of this dissolution episode however were not so extreme in the marl that was deposited at the same time in slightly deeper water in northwestern denmark evidence from cretaceous tertiary boundary sequences in denmark and elsewhere suggests that no single catastrophe can account for the major biotic extinctions that occurred at the end of the cretaceous period the primary causal factors of the terminal cretaceous extinction event appear to be drastic global sea level regression occurring simultaneously with extensive volcanism on land and a strong pulse of calcite dissolution in ocean surface waters

fires at the k t boundary carbon at the sumbar turkmenia site abstract we have measured carbon at the marine k t boundary site sm 4 at the sumbar river in turkmenia ussr which has an undisturbed ir profile and the largest known ir anomaly 580 ng cm 2 twenty samples ranging from 100 to 100 cm were analyzed to determine the concentration and 13 c of elemental carbon and kerogen using a cr 2 o 7 oxidation method wolbach and anders 1989 to resolve these components the samples were unusually complex containing 3 kerogen components of distinctive ir spectra and half lives in cr 2 o 7 200 100 and 60 h in addition to soot and charcoal from the k t fire the elemental c has 13 c 25 96 0 6 l o close to the mean for 11 k t sites 25 8 0 6 11 mg cm 2 the first of the 2 kerogen components 13 c 22 8 which dominates in the cretaceous appears to come from detrital carbonaceous shale the second 13 c 27 7 o occurs only in the boundary clay and may come from land plant material swept to sea or phytoplankton grown in the presence of excess light co 2 from fires on land the great thickness of the boundary clay 10 5 cm permits determination of the compositional profiles with unprecedented spatial resolution ir and shocked quartz both representing impact ejecta rise sharply at the boundary peak in the basal layer and then decline soot and total elemental c show a similar spike in the basal layer but then rise rather than fall peaking at 7 cm apparently fires started before the basal layer had settled implying that ignition and spreading of major fires became possible if not immediately then very soon after the impact there is no basis for claims by hansen et al 1987 that k t carbon black first occurs 3 5 m below the danish k t boundary and has 13 c of 27 4 to 27 8 o reexamination of a sample from nye klov shows that these authors confused kerogen with carbon black

contemporary state of stress and neotectonic deformation in the carpathian pannonian region the carpathian pannonian region has been characterized by a two stage tectonic evolution since the beginning of the neogene during early and mid miocene the lateral eastward escape of the pannonian fragment caused thrusting and folding in the outer carpathian mountains south and westward directed subduction of the eurasian lithosphere calc alkaline volcanism along the inner carpathian bend and localized fault controlled subsidence of basins in the pannonian region this style of tectonic deformation ceased by the end of the mid miocene the neotectonic stage began in the late miocene it is characterized by differential regional subsidence with maximum rates in the west and east and minimum rates in the centre of the pannonian basin further characteristics of the central pannonian basin are the anomalous high heat flow values the thin crust and lithosphere the neotectonic stress field has been determined by in situ stress measurements the analyses of borehole breakouts and fault plane solutions of earthquakes in situ stress measurements by the doorstopper and triaxial strain cell methods indicate high compressional stresses in the western pannonian basin with its maximum in wnw ese to nw se direction in contrast to its western part the central pannonian basin shows tensional stresses near the earth s surface with maximum tension in the same wnw ese direction borehole breakout data indicate a general wnw ese orientation of maximum horizontal stress in the western part of the pannonian basin and in the eastern part as well whereas in the central pannonian basin this direction is the preferred azimuth of minimum horizontal stress it is suggested that the neotectonic deformations and stresses in the carpathian pannonian region have a sublithospheric origin asthenospheric convection with an upwelling mantle flow below the centre of the pannonian basin and downwelling flows along the cold lithospheric roots below the eastern carpathians and the alps seems to be most plausible accordingly the relative uplift of the central pannonian basin the high heat flow and also the tensional stresses are explained as the surface expression of an upstreaming branch of a localized convection cell below the pannonian basin

the effects of temperature dependent viscosity on mantle convection with the two major phase transitions abstract we have studied the consequences of including temperature dependent viscosity in mantle convection with both of the major phase transitions included we have employed a two dimensional cartesian model in a box of aspect ratio four the time dependent equations have been cast within an extended boussinesq framework in which each of the phase changes has been simulated by an enhanced thermal expansivity both latent heat release and viscous dissipation have been considered two different forms of temperature dependent viscosity the arrhenius and the frank kamenetzky forms have been employed we have considered both models with a constant temperature equilibrium boundary condition at the core mantle boundary and models with a time varying non equilibrium temperature boundary condition from cooling of the core the effects of temperature dependent viscosity are to enhance the tendency of the convective system to be layered because of the influences of the latent heat release and the mechanical heating on creating a sharp viscosity drop across the transition zone we have found that there is a non linear coupling between latent heat release from the interaction of the hot plume with the endothermic phase transition and the temperature dependent viscosity this positive feedback process can give rise to the production of very high temperatures in the transition zone which may cause melting in the region between the two major phase transitions flushing events produced by the non equilibrium boundary condition are much more dramatic than those generated by the equilibrium boundary condition the non linear coupling between latent heat release and temperature dependent viscosity is also stronger in the non equilibrium models comparison with models without the presence of latent heat release and viscous dissipation shows that the models without the heating terms produce a greater degree of mass flux across the endothermic phase boundary because of the lack of the viscosity stratification in the interior of the convective flow results of this moded can be applied to other types of endothermic phase transition found in the lithosphere or to the spinel perovskite transition in the deep portion of the martian mantle

the role of lithospheric phase transitions on seafloor flattening at old ages abstract uplift of old ocean seafloor relative to the familiar t 1 2 curve is generally presumed to be a consequence of an additional heat source at the base of the oceanic lithosphere several geodynamical mechanisms have been invoked to explain this behavior we have taken a petrological rather than geodynamical approach and considered whether the causes of the flattening can be found within the conductive portion of the lithosphere where viscous creep is insignificant accordingly self consistent calculations of the phase assemblages of several candidate mantle compositions have been performed subject to the constraint of experimentally determined calorimetric measurements we have found that in general for temperatures below around 800 c and pressures greater than 10 kbar the phase change from spinel peridotite to garnet peridotite has a significantly negative clapeyron slope which could cause some amount of uplift of old seafloor the transition for representative thermal profiles should be depressed from about 35 km at 40 ma to 45 km at 160 ma since spinel peridotite is of lower density the net effect is to raise the seafloor topography the extent of the uplift which depends primarily on the al 2 o 3 content of the peridotite and on the effective thermal diffusivity of the lithosphere should be on the order of 140 220 m

magnetostratigraphy and paleoenvironmental events recorded in a late cenozoic sedimentary succession in huaibei plain east china abstract the thick late cenozoic deposits of huaibei plain provide a record of paleoenvironmental changes in the northern and southern transitional zone of eastern china and of the evolution of the asian monsoon system a detailed magnetostratigraphic study of the huainan hn 481 45 m deep drill core from the center of huaibei plain reveals a magnetic polarity sequence from chron c4n 2n to chron c1n spanning the interval from 8 ma to the present it is the first reliable magnetostratigraphic chronology for the late miocene to present for huaibei plain the stratigraphic sequence contains three intervals characterized by major peaks in magnetic susceptibility at depths of 58 8 m 108 0 m and 312 4 m which can be used as isochronous marker beds for regional stratigraphic correlations in huaibei plain major changes in sediment grain size occur at 7 0 and 1 7 ma corresponding to pronounced environmental shifts which we consider reflect the respective strengthening of the south asian summer monsoon and east asian winter monsoon at these times

seawater sr isotopes at the cretaceous tertiary boundary abstract seawater 87 sr 86 sr values increase abruptly by 28 10 6 across the cretaceous tertiary boundary ktb this small but rapid shift is superimposed on the larger scale structure of the seawater sr isotope curve the time scale of radiogenic sr addition appears to be too rapid to reconcile with sources associated with volcanism and we show that the amount of sr required to produce even this small increase is too large to be derived from 1 a kt bolide of the size constrained by the ir anomaly 2 continental crust ejecta from the impact of such a bolide 3 soot from global wildfires initiated by an impact or 4 any combination of these sources the probable source of the radiogenic sr is enhanced continental weathering but the high rate of increase appears to rule out processes such as sea level regression glaciation or tectonism a plausible mechanism for rapid addition of radiogenic sr to the oceans is enhanced weathering associated with globally distributed acid rain ph 1 which is a proposed by product of a bolide impact 51 epsl vol 83

late cretaceous and paroxysmal cretaceous tertiary extinctions the various geological signatures at cretaceous tertiary time including iridium and other associated elements microspherules and shock deformation features are compatible with the suggestion that the transition is marked by a period of intense volcanism the volatile emissions from this volcanism would lead to acid rain reduction in the alkalinity and ph of the surface ocean global atmospheric temperature changes and ozone layer depletion these environmental effects coupled with those related to the major sea level regression of the late cretaceous provide the framework for an explanation of the selective nature of the observed extinction record

trace element anomalies at the mississippian pennsylvanian boundary in oklahoma and texas trace element abundance anomalies have been found at the mississippian pennsylvania boundary at sites in oklahoma and texas where the boundary has been precisely located on the basis of an abrupt change in conodont diversity and species composition enriched elements include osmium indium platinum chromium most chalcophiles rare earths and uranium the anomalies are more intense e g os 4 ppb ir 0 38 ppb pt 6 ppb cr 12000 ppm u 380 ppm and peisist through a thicker interval at the south central texas locality than in oklahoma and in bolh locations the anomalies are associated with an increase in phosphate content of the rocks there is no tangible evidence of an asteroid or comet impact source for the excess pt group elements and fauna crisis the cause of the elemental enrichments and the biological disturbance may possibly be related to a change in the ocean chemistry of the paleozoic seaway such as increased upwelling stagnation or nearby submarine volcanism

phase changes and thermal subsidence in intracontinental sedimentary basins summary a model of tectonic subsidence is developed to explain the late acceleration of subsidence observed in some intracratonic sedimentary basins the proposed mechanism combines the effect of thermal contraction of an initially hot lithosphere with the effect of a subcrustal phase transformation that moves under changing pressure and temperature conditions the subsidence following a sudden change in temperature at the base of the lithosphere is calculated the calculations show that 1 phase changes if present and activated contribute substantially to the subsidence of sedimentary basins 2 because the effect of phase change is delayed subsidence accelerates after a time on the order of 20 myr and 3 the duration of the subsidence is on the order of 100 to 150 myr during the late stages of subsidence the phase change is the dominating mechanism an application to the michigan basin is presented the calculated sediment accumulation history fits the record well when the effect of sea level changes is included in the model

the effects of the spinel garnet phase transition on the formation of rifted sedimentary basins summary we have examined the effects of the spinel garnet phase transition on subsidence of extensional sedimentary basins for a constant positive clapeyron slope dp dt the phase boundary moves downwards in the syn rift and upwards in the post rift phase for a non linear clapeyron curve dp dt 0 above 900 c and dp dt 0 below 900 c theory predicts for the reaction of the spinel garnet phase transition the direction of phase boundary movement is dependent on the stretching factor the position of the clapeyron curve and the lithospheric thickness a smaller syn rift and larger post rift subsidence are predicted for a deeper phase boundary and a thicker lithosphere the model with a non linear clapeyron curve is applied to the subsidence histories of a young extensional basin gulf of lion and an old continental margin eastern canada the observed syn rift uplift and the larger post rift subsidence can be reasonably explained by this model where the optimum depth of the phase boundary for eastern canada 90 km is consistent with the estimate from seismic observations and is larger than that for the gulf of lion 50 km the depth of the spinel garnet phase boundary is sensitive to the composition of mantle rocks and increases with the extraction of basaltic components from the lithosphere compatible with our result that the phase boundary is deeper for an older and thicker lithosphere thus the surface movement associated with the rifting for these areas may reflect the chemical evolution of the continental lithosphere

subsidence of the paris basin and its bearing on the late variscan lithosphere evolution a comparison between plate and chablis models abstract in this study two different models for the lithospheric thermal evolution are used to predict the long term subsidence of the paris basin the plate or chablis models have a constant temperature or constant heat flow at the base of the lithosphere these boundary conditions yield a different thermal relaxation time for a given equilibrium lithospheric thickness the final thermal steady state does not depend on the boundary condition fixed at the base of the lithosphere the thermal evolution of the lithosphere is computed taking into account temperature and pressure dependent thermal characteristics metamorphism in the crust top crustal erosion and phase transition in the mantle spinel garnet the long term tectonic subsidence of the paris basin constrains the mesozoic thermal evolution of the lithosphere and the amplitude of the initial thermal anomaly the chablis model including the effects of spinel garnet phase transition predicts present day lithospheric thickness in better agreement with observation than the plate model the chablis model involves a basal heat transfer of 37 mw m2 similar to the value beneath oceanic plates the long term subsidence of the paris basin obviously results from the decay of a thermal anomaly initiated during late variscan times we first assume that at 300 ma the lithosphere was less than 150 km thick and the thickened crust started returning to its present thickness 35 km by uniform lithospheric extension the subsidence data can be explained by short stephano autunian as well as long stephano triassic lasting extension these hypotheses both implicitly refer to extensional collapse of the variscan belt however the evolution of the variscan belt from 350 to 300 ma may have involved almost no crustal thickening but an anomalously thin lithosphere ca 55 km the corresponding thermal evolution of the lithosphere is computed using the chablis model with an initially high heat transfer of 61 mw m2 at the base of the base of the lithosphere at 280 ma this heat transfer is tuned to 40 mw m2 leading to lithospheric cooling from 280 to 30 ma which can also explain the paris basin subsidence for the short lasting extension hypothesis the late variscan lithospheric evolution predicted with the chablis model yields paleogeography magmatism and retrograde p t t paths in good agreement with those reported from areas around the paris basin conversely the plate model does not allow mantle melting during the end of carboniferous and permian times the long lasting extension hypothesis does not explain the paleogeographic evolution the timing of magmatism and retrograde p t paths showing isothermal decompression the hypothesis with a strong lithospheric delamination cannot be ruled out however it is less plausible than the extensional collapse hypothesis

the extraterrestrial component in marine sediments description and interpretation the influx of extraterrestrial matter to the earth is dominated by two size fractions sub millimeter interplanetary dust and impacting asteroids and comets over geologic time the major contribution of extraterrestrial matter is from the largest impactors interplanetary dust is largely vaporized by atmospheric entry but some surviving material is found in sediments as cosmic spherules impacting asteroids and comets can produce fallout layers either regionally or globally which may contain a significant fraction of meteoritic material as well as shock metamorphosed and shock melted terrestrial material iridium is a sensitive tracer of extraterrestrial matter although high concentrations of ir in marine sediments probably have an extraterrestrial origin they do not necessarily indicate the presence of a major impact event of the thousands of impact horizons which must be resolvable in the sedimentary record to date only seven probable impact horizons have been identified in the entire phanerozoic the marine geochemistry and occurrence of ir is still poorly understood but the thousands of ir analyses performed in the last several years have demonstrated that the global occurrence high ir 10 ng g concentrations in cretaceous tertiary boundary sediments is a truly anomalous phenomenon

iridium and other platinum group elements as geochemical markers in sedimentary environments iridium anomalies are commonly used as indicators of extraterrestrial processes which caused global extinctions however platinum group element pge anomalies may result from several different processes both syn and postdepositional acting separately or jointly the sources and processes of enrichment involved are extraterrestrial asteroid comet cosmic dust impact ejecta volcanogenic pge rich condensates precipitation from seawater low sedimentation rate anoxic conditions microbial concentration dissolution reprecipitation exhalative hydrothermal leaching transport and precipitation at a redox boundary geochemical characteristics of pge and accompanying elements in sediments can be used to identify the contributions of some specific sources and processes 1 extraterrestrial chondritic ru ir and os ir ratios low 187os 186os ratio high ir compared to pt pd and au 2 volcanogenic high iridium and se as sb and in contents 3 exhalative hydrothermal high au pd and pt relatively low ir contents 4 precipitation from seawater high pt and low pd contents also high cd and ag contents under anoxic conditions and 5 reduction from intermediate and low temperature solutions high pge and cu or mo u ni contents

structural modelling of regional depth profiles in the voring basin implications for the structural and stratigraphic development of the norwegian passive margin interpretation of key regional seismic lines across the voring basin has identified a number of seismic megasequences that have provided a framework for further basin analysis these megasequences have been classified as pre syn and post rift with respect to the late cretaceous early tertiary phase of extension forward and reverse structural and stratigraphic modelling of two of these regional lines has been performed in an attempt to test and validate the structural and stratigraphic interpretations of the syn and post rift megasequences reverse thermal subsidence modelling flexural backstripping of these lines to the base tertiary was performed using a laterally varying beta factor across the sections a number of features on each of the depth profiles are considered to be regional subaerial datums for the backstripped sections the backstripped templates were then forward modelled using the flexural cantilever model the forward models calculate a laterally varying beta profile along their length which can be compared to the reverse models thereby ensuring an internally consistent structural model between both the reverse and forward modelling techniques the modelling work has highlighted the existence of a significant discrepancy between the extension measured from the forward models fault heaves and the extension needed to produce geologically acceptable backstripped profiles this observation is of particular importance as it may place a significant constraint on the mode of lithospheric extension that can be used to explain the evolution of the norwegian continental margin

weathered spheroids in a cretaceous tertiary boundary shale at woodside creek new zealand three types of spheroids are found in the iridium rich shale layer at the cretaceous tertiary boundary at woodside creek south island new zealand two of the types a and b consist mainly of microcrystalline goethite whereas the third type f contains an appreciable quantity of hematite there is a strong correlation between the iridium content of the spheroids and their matrices and the concentrations of other siderophiles such as chromium cobalt iron and nickel a similar pattern was observed for the chalcophiles arsenic copper and zinc it is suggested that spheroids of types a and b are weathering products derived from diagenetic pyrite clumps and that type f which contains hematite represents a further stage in the weathering process and was derived from goethite the original pyritic material could have readily scavenged arsenic and other siderophiles from the sea water column under organic rich reducing conditions at the end of the cretaceous and the siderophiles including iridium could have been incorporated into the original meteoritic material it was concluded therefore that types a b and f spheroids are weathering products and do not represent original meteoritic material

tectonic subsidence of the lomonosov ridge the cenozoic sedimentary record revealed by the integrated ocean drilling program s arctic coring expedition acex to the lomonosov ridge microcontinent in 2004 is characterized by an unconformity attributed to the period 44 18 ma according to conventional thermal kinematic models the microcontinent should have subsided to 1 km depth owing to rifting and subsequent separation from the barents kara sea margin at 56 ma we propose an alternative model incorporating a simple pressure temperature p t relation for mantle density using this model we can explain the missing stratigraphic section by post breakup uplift and erosion the pattern of linear magnetic anomalies and the spreading geometry imply that the generation of oceanic crust in the central eurasia basin could have been restricted and confined by non volcanic thinning of the mantle lithosphere at an early stage ca 56 40 ma in response to a rise in temperature the mantle mineral composition may have changed through breakdown of spinel peridotite and formation of less dense plagioclase peridotite the consequence of lithosphere heating and related mineral phase transitions would be post breakup uplift followed by rapid subsidence to the deep water environment observed on the lomonosov ridge today

extraterrestrial spherules in glacial sediment from the transantarctic mountains antarctica structure mineralogy and chemical composition silicate and glassy spherules of probably extraterrestrial origin have been discovered in glacial till from localities in the beardmore glacier region transantarctic mountains antarctica these spherules are similar in structure appearance and composition to spherules found in deep sea sediments and in greenland most spherules have a shiny and smooth dark surface while some show a characteristic brickwork pattern composed of intergrown olivine crystals and magnetite dendrites several spherules have been sectioned after trace element analysis by inaa the interiors of the spherules contain olivine crystals of varying size predominantly of forsteritic composition set in a glassy fe rich and mg poor matrix the matrix contains very fine grained dendritic magnetite crystals and rare accessory wustite sem and microprobe studies show that most of the spherules have not experienced significant weathering trace element analyses for individual particles confirm the extraterrestrial nature of the spherules they have undifferentiated chondritic rare earth element ree patterns and are enriched in ir and other siderophile elements with ir contents between 10 and 1 000 ppb some spherules have experienced loss of volatile elements and some differentiation of siderophile elements e g ni the antarctic spherules which might be micrometeorites that melted during passage through the atmosphere or ablation spherules are amore significant addition to the collection of extraterrestrial microparticles of key importance is their high abundance in the sediment and the fact that they have been concentrated by glacial action less

back arc rifting initiated with a hot and wet continental lithosphere abstract how back arc rifting is initiated is studied by inferring the tectonic force and strain rate regimes from published subsidence data for nine back arc basins in the european tethyan belt and in the western pacific the results show that rifting only begins after some particular magnitude of tectonic force is reached this implies that subduction has already progressed to a point such that the total negative buoyancy of the slab becomes so large that the relevant tectonic force exceeds its bending strength once back arc rifting has been initiated the force required to maintain it gradually decays this indicates that the processes governing back arc extension are fast enough that lithosphere weakening due to an increase in geothermal gradient exceeds strengthening due to the thermal relaxation and crustal thinning this is achieved given currently accepted estimates for the maximum force 7 tn m if the thickness of the thermal lithosphere is significantly less than 125 km and if the lithosphere has a wet rheology meaning that a weak continental lithosphere is required for the initiation of back arc rifting the presence of significant strength in the uppermost mantle is not precluded but it is not required

187os 186os in marine manganese nodules and the constraints on the crustal geochemistries of rhenium and osmium during their formation marine manganese or more properly ferromanganese oxide nodules include and commonly concentrate elements dissolved in seawater this attribute makes them especially valuable recorders of the isotopic composition of elements supplied to the oceans having radiogenic isotopes for these can be used in exploring crustal composition and evolution1 5 from measurements of 187os 186os ratios in marine manganese nodules and comparison with the 143nd 144nd ratios of the same nodules we have calculated the os isotopic composition and the osmium and rhenium concentrations of the weatherable portion of the continental crust together with the proportions of ultra mafic and granitic rock that constitute this crustal mass