87sr86sr in late proterozoic carbonates evidence for a mantle event at 900 ma ago abstract strontium isotopic measurements were made on late proterozoic carbonates from west african craton comparison of samples with acceptable trace element patterns with coeval data from southern africa and with the published australian results suggests that the 87 sr 86 sr ratio of the late proterozoic sea water evolved in the following manner about 0 7075 at 1000 50 ma 0 7056 to 0 7074 at 900 50 ma 0 7068 to 0 7091 0 7106 at 800 50 ma 0 7074 to 0 7077 at 700 50 ma and 0 7076 to 0 7089 0 7096 at 600 50 ma ago the variations are comparable in magnitude and frequency to those described previously for the phanerozoic strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of sr into late proterozoic oceans was of comparable isotopic composition to its present day counterpart 0 711 consequently the non radiogenic 87 sr 86 sr value at 900 50 ma ago signifies a large flux of mantle strontium into the ocean at this time because the present time resolution is only about 75 25ma additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed late proterozoic sea water curve

land plant evidence compatible with gradual not catastrophic change at the end of the cretaceous field study of the fossil and sedimentary record across the cretaceous tertiary k t boundary in wyoming and montana has been combined here with a reassessment of the published record of terrestrial palynomorphs to test recent hypotheses that a universal biotic catastrophe caused by an asteroid1 3 cometary impact4 or a supernova5 terminated the cretaceous evidence from land plants is particularly critical because plants form the base of the terrestrial food chain thus massive disruptions of the earth s vegetation by the postulated effects of a cosmic disaster blocking of solar radiation for several years by a dust cloud1 severe atmospheric heating1 4 6 7 or ionizing radiation5 would have an amplified effect on the land fauna however i report here that the geographically uneven and generally moderate levels of extinction and diversity change in the land flora together with the non synchroneity of plant and dinosaur extinctions contradict hypotheses that a catastrophe caused terrestrial extinctions

depositional environment and mechanism of preservation of microfossils upper proterozoic bitter springs formation australia microfossils preserved in black chert nodules from the upper proterozoic bitter springs formation of australia represent the preserved remnants of cyanobacterial communities that inhabited a series of nonmarine saline lakes and ponds groundwaters of halite salinity pickled the microorganisms and thus inhibited their early bacterial degradation these same groundwaters were saturated in silica so that upon evaporation the silica and halite precipitated and formed a series of chert concretions nodules and bipyramidal quartz crystals where the chert nodules formed in areas of buried cyanobacterial mats further degradation of the buried sheaths and cells was arrested and the microorganisms were preserved

thermal effects of normal faulting during rifted basin formation 2 the lugano val grande normal fault and the role of pre existing thermal anomalies abstract we investigate the thermal consequences of rift related normal faulting and compare the results with a well studied natural example the lugano val grande normal fault southern alps only limited heating of the crust is caused by lithospheric thinning in the simple but realistic situation where heat conduction is substantially faster than heat advection no major thermal disturbance is associated with the downward movement of the hanging wall radiometric ages and fault rocks associated with the lugano val grande normal fault demonstrate that cooling rather than heating affected the crust during normal faulting this pattern is not compatible with such a simple numerical model and is explained by a waning thermal anomaly induced by a magmatic intrusion immediately preceding or overlapping with the first stages of normal faulting the magmatic body must have been emplaced at depths greater than 15 18 km and probably started to cool in the carnian i e few million years before the onset of normal faulting along the lugano val grande fault

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

tectono stratigraphic modelling of the north sicily continental margin southern tyrrhenian sea abstract a two dimensional numerical modelling that simulate the kinematic and thermal response of the lithosphere to thinning was used for the quantitative reconstruction of the late neogene to recent times tectonic and stratigraphic evolution of the north sicily continental margin southern tyrrhenian sea the numerical study of the evolution of the north sicily margin builds on the crustal image and kinematic interpretation of the margin obtained by pepe et al tectonics 19 2000 241 on the basis of seismic data and gravity modelling tectonic modeling indicate that different segments of the margin were undergoing different vertical movements which are mainly expression of the rifting and thinning of the lithosphere occurred during tectonic evolution of the southern tyrrhenian sea a prediction of the pre rift basement topography and the moho along the margin converges to a value of 6 5 km for the depth of necking and a temperature dependent eet 500 isotherm the model fails to reproduce the morphology of the solunto high confirming its non extensional origin a polyphase evolution is required to reproduce the observed syn and post rift stratigraphy during the first rifting stage between 9 and 5 ma crustal thinning factors reach maximum values of 1 27 in the cefalu basin a similar value is predicted for the subcrustal thinning around 60 km nne of the profile margin crustal thinning factors increase during the second rifting stage from 4 to 2 ma and reach values of 2 and up to 3 5 in the cefalu basin and in the continent oceanic transition zone respectively similarly subcrustal lithospheric thinning factors reach values up to 2 5 in the distal sector of the margin an uplift of more than 100 m is predicted for the north sicily shelf and surrounding onshore areas during the post rift stage the evolution of thermal structure with time is very sensitive to the partial thinning factors describing the evolution of the thinning itself during time the lithosphere preserved part of its strength during extension the effective elastic thickness eet along the margin through time is 24 km at the onset of rifting and reaches values less to 8 km during the second rifting stage in the northeastern end of the margin

precise age of bangiomorpha pubescens dates the origin of eukaryotic photosynthesis although the geological record indicates that eukaryotes evolved by 1 9 1 4 ga their early evolution is poorly resolved taxonomically and chronologically the fossil red alga bangiomorpha pubescens is the only recognized crown group eukaryote older than ca 0 8 ga and marks the earliest known expression of extant forms of multicellularity and eukaryotic photosynthesis because it postdates the divergence between the red and green algae and the prior endosymbiotic event that gave rise to the chloroplast b pubescens is uniquely important for calibrating eukaryotic evolution however molecular clock estimates for the divergence between the red and green algae are highly variable and some analyses estimate this split to be younger than the widely inferred but poorly constrained first appearance age of 1 2 ga for b pubescens as a result many molecular clock studies reject this fossil ex post facto here we present new re os isotopic ages from sedimentary rocks that stratigraphically bracket the occurrence of b pubescens in the bylot supergroup of baffin island and revise its first appearance to 1 047 0 013 0 017 ga this date is 150 m y younger than commonly held interpretations and permits more precise estimates of early eukaryotic evolution using cross calibrated molecular clock analyses with the new fossil age we calculate that photosynthesis within the eukarya emerged ca 1 25 ga this date for primary plastid endosymbiosis serves as a benchmark for interpreting the fossil record of early eukaryotes and evaluating their role in the proterozoic biosphere

upper cretaceous paleocene biostratigraphy and magnetostratigraphy hell creek and tullock formations northeastern montana fossils from the hell creek and tullock formations in northeastern montana provide detailed documentation of terrestrial faunal and floral evolution during latest cretaceous lancian and early paleocene puercan time here the replacement of lancian faunas by those of puercan age most obviously signaled by the extinction of dinosaurs and the changes in pollen floras sometimes used to mark the cretaceous tertiary boundary occurred during a period of reversed magnetic polarity paleontological correlations suggest that dinosaur extinction and the change in pollen floras took place in the red deer valley area alberta during the same period of reversed polarity furthermore also on the basis of paleontological correlations the extinction of dinosaurs in the san juan basin new mexico appears to have occurred either during the same period of reversed polarity or possibly during the preceding period of normal polarity

microfossils in 2000 ma old cherty stromatolites of the franceville group gabon abstract a well preserved microbial assemblage has been found in paleoproterozoic cherty stromatolites from the 2000 ma old franceville group gabon this assemblage contains coccoids and star like forms filaments and budding tubular forms these various forms are mostly of cyanobacterial affinity and among them nostocalean akinetes aerchaeoellipsoides are reported for the first time from rocks older than the mesoproterozoic the most noteworthy discovery in the franceville biota is the gunflint type association huroniospora gunflintia eoastrion archaeorestis known also without archaeorestis in other paleoproterozoic assemblaes throughout the world the present assemblage comprises 13 taxa consisting of 8 species belonging to 10 genera and 2 unnamed forms it is dominated by gunflintia filaments regarded as a mat bulding community and by huroniospora coccoids probably mat dwellers and or plankters the rare and large forms such as akinetes of archaeoellipsoides were allochthonous and presumably planktonic

violent volcanism stagnant oceans and some inferences regarding petroleum strata bound ores and mass extinctions past changes from a well mixed aerated ocean to a stratified stagnant ocean are indicated by decreasing 13c of limestone opposite to prior conclusions as well as by increasing 34s of marine sulfates the proposed stagnant ocean model includes a warm co2 enriched atmosphere and a bathyal to abyssal marine system dominated by bacterial components in the food web and by consequent light carbon in organic material and biogenic carbonates culminations of prolonged stagnant episodes correspond with marine faunal extinctions of late permian and late cretaceous time and the protracted changes and correlations provide evidence against any hypothesis of mass extinction by sudden worldwide catastrophe including the asteroid impact hypothesis the cretaceous stagnant ocean taken as the prime example is attributed to climatic warming triggered by volcanic co2 greenhouse effect and several feedbacks including decreased earth albedo and increased sinking of warm evaporite brines instead of aerated polar waters marine extinctions are attributed to upward expansion of the oxygen minimum zone and to catastrophic mixing of surface waters with poisonous sulfidic waters of the deeps the stagnant ocean provided a counterbalance between deep reduction and shallow oxidation conditions that favored maximal formation of black sulfidic shales protopetroleum and sedimentary sulfide ores and shallow to mid depth barite phosphorite iron ore cherty iron formation and sulfate bearing red beds trace elements concentrated in stagnant ocean sediments include chalcophile and pt group metals negating claims that ir provides a unique fingerprint of meteorite impact and cosmic accretion

heat flow in the balearic and tyrrhenian basins western mediterranean we present the results of three detailed heat flow surveys which are used to investigate the variations of heat flow and age of the balearic and tyrrhenian basins in the western mediterranean analysis of 12 measurements within a 10 km radius from 40 01 4 55 e in the balearic abyssal plain shows a mean heat flow of 92 10 mw m 2 after correction for the effects of sedimentation this value agrees well with the predictions of plate cooling and high extension stretching models for crust of the late oligocene age proposed for the basin a similar survey with 18 measurements around 40 16 n 11 19 e in the western tyrrhenian gives a flux of 134 8 mw m 2 while the third survey of 26 measurements in the southern tyrrhenian abyssal plain at 37 17 n 12 58 e yields a slightly higher heat flow of 151 10 mw m 2 these values are within the range predicted by simple plate cooling models for the late miocene ages of the deep tyrrhenian basin thus our observations suggest that although the mode of crustal formation of these deep marginal basins is less well defined than that of the major ocean basins the thermal signature is similar also the trend of increasing heat flow from west to east through the balearic and tyrrhenian basins is in agreement with models of the formation of the western mediterranean behind an eastwardly migrating trench system in all three areas the measured flux shows significant local variability in the two westernmost surveys this can be attributed to the presence of buried high conductivity salt structures but in the third area se tyrrhenian the magnitude of the variations is several times greater than can be attributed to steady state thermal refraction alone instead evidence exists for localized hydrothermal activity linked to the small topographical relief within the survey area

rifted margin formation in the south tyrrhenian sea a high resolution seismic profile across the north sicily passive continental margin a new 150 km long seismic line across the continental margin of north sicily has been acquired and interpreted the overall structure of the margin is controlled by extension which caused crustal thinning and widespread normal faulting two main thinned zones are observed in the south in correspondence with the cefalu basin and farther to the north at the continent ocean transition zones of thinned crust coincide with zones of intense normal faulting extension began in late tortonian times and caused the opening of the cefalu basin controlled by a northward dipping listric fault messinian stretching affected most of the future margin and provoked a widening of the cefalu basin and normal faulting in the north following a phase of relative quiescence in the early pliocene renewed extension determined further opening of the cefalu basin and subordinate normal faulting in the north here however the record is unclear because of the emplacement of the calc alkaline sisifo volcano with associated volcanoclastic deposits breakup took place in the late pliocene and was followed by the deposition of postrift pleistocene sediments at the lithospheric scale the sites of extension thinning did not migrate during rifting on the smaller scale on the contrary the cefalu basin displays a remarkably systematic pattern of migration toward the foot wall of the listric fault which controlled the opening of the basin the spacing of 4 6 km between faults is also quite systematic elongation experienced by the continental part of the margin presently 97 km has been derived by comparing the present day and the preextensional lengths and is 10 km the corresponding strain rate is 5 10 16 s 1

microbial metazoan reefs of the terminal proterozoic nama group c 550 543 ma namibia thrombolite and stromatolite reefs occur at several stratigraphic levels within the terminal proterozoic nama group c 550 543 ma of central and southern namibia the reefs form integral parts of several carbonate platforms within the nama group including the kuibis platform of the northern nama basin zaris subbasin and huns platform witputs subbasin of the southern nama basin the reefs are composed of both thrombolites and stromatolites that form laterally continuous biostromes isolated patch reefs and isolated pinnacle reefs ranging in scale from a metre to several kilometres in width in the majority of cases the reefs occur stratigraphically as an integral facies within the transgressive systems tracts of sequences making up the kuibis and huns platforms this suggests that a regime of increasing accommodation was required to form well developed reefs though reefs also occur sporadically in highstand systems tract settings within a given transgressive systems tract a regime of increasing accommodation through time favours the transition from sheet like biostromal geometries to more isolated patch and pinnacle biohermal geometries similarly increasing accommodation in space such as a transect down depositional dip shows a similar transition from more sheet like geometries in updip positions to more isolated geometries in downdip positions reefal facies consist of thrombolitic domes columns and mounds with well developed internal clotted textures in addition to stromatolitic domes columns and mounds with crudely to moderately well developed internal lamination stromatolites are better developed in conditions of relatively low accommodation and updip locations under conditions of higher current velocities and greater sediment influx thrombolites are better developed in conditions of relatively high accommodation and low sediment influx both types of microbialites are intimately associated with the first calcifying metazoan organisms which may have attached themselves to the sediment surface or otherwise lived within sheltered depressions within the rough topography created by ecologically complex mats the appearance of thrombolitic textures during terminal proterozoic time is consistent with colonization of cyanobacterial mats by higher algae and metazoans which would have been an important process in generating clotted textures fabrics in the nama thrombolites are well preserved and show evidence of thrombolitic mesoclots being overgrown by fibrous marine carbonate interpreted as former aragonite this was followed by emplacement of geopetal micrite fills and precipitation of dolomite as an isopachous rim cement followed by occlusion of remaining porosity by blocky calcite spar

the moho in extensional tectonic settings insights from thermo mechanical models the lithospheric memory is key for the interplay of lithospheric stresses and rheological structure of the extending lithosphere and for its later tectonic reactivation other important factors are the temporal and spatial migration of extension and the interplay of rifting and surface processes the mode of extension and the duration of the rifting phase required to lead to continental break up are to a large extent controlled by the interaction of the extending plate with slab dynamics the finite strength of the lithosphere has an important effect on the formation of extensional basins this applies both to the geometry of the basin shape as well as to the record of vertical motions during and after rifting we demonstrate a strong connection between the bulk rheological properties of europe s lithosphere and the evolution of some of europe s main rifts and back arc systems the thermo mechanical structure of the lithosphere has a major impact on continental break up and associated basin migration processes with direct relationships between rift duration and extension velocities thermal evolution and the role of mantle plumes compressional reactivation has important consequences for post rift inversion borderland uplift and denudation as illustrated by poly phase deformation of extensional back arc basins in the black sea and the pannonian basin region

tectono stratigraphic modelling of the sardinian margin of the tyrrhenian sea rifting of the east sardinia passive margin was characterised by a southeastward migration of the site of extension through time in the first stage of rifting 9 7 ma lithospheric thinning was mainly localised near sardinia crustal and subcrustal thinning factors averaged across the whole transect were 1 12 and 1 5 respectively at 7 ma i e immediately before the onset of the messinian salinity crisis the basin floor was at depths of up to 1000 m in the west and less than few hundred metres in the east during the second stage of rifting between 7 and 5 ma the site of main extension was located in the central part of the future margin with average crustal and subcrustal thinning factors of 1 18 and 1 63 respectively at 5 ma after the termination of the messinian salinity crisis the morphology of the basin floor had changed substantially with depths in general between 700 and 1000 m extension during the last rifting stage before oceanic crust formation at 3 5 ma was mainly concentrated in the southeastemmost part of the profile average crustal thinning factors increased dramatically to i 95 while the subcrustal one remained almost unchanged 1 64 the analysis of temporal changes of thinning factors shows that the opening of the tyrrhenian oceanic basin can be adequately explained with a depth dependent pure shear geometry with crustal and subcrustal thinning being accommodated on the same vertical column subcrustal thinning factors in the initial stages of rifting were higher than the crustal ones but the two become comparable approaching break up the regional compensation of changing loads coupled with a very deep depth of necking is consistent with the observed subsidence trend and basin stratigraphy

a seismic refraction experiment in the tyrrhenian sea summary we present the results of two 100 km long reversed seismic refraction lines shot in the tyrrhenian sea using explosive sources and oceanbottom receivers the results have been interpreted using ray tracing through laterally varying structures the more easterly line t2 was shot in the central part of the tyrrhenian abyssal plain in a water depth of 3 5 km the crust here is shown to be oceanic in character but to exhibit significant lateral heterogeneity the upper part of the crust layer 2 contains a steep velocity gradient from about 4 to over 6 km s l at a depth of typically 2 km the lower part of the crust layer 3 contains a smaller velocity gradient from 6 6 to 6 9 km s l at the base of the crust total crustal thickness is between 6 and 8 km the other line t1 carried out in the cornaglia basin in 2 7 km of water reveals a more complicated velocity structure we conclude that its southern half is underlain by oceanic crust similar to that observed at t2 however its northern half appears to be underlain by crust of similar thickness but in which seismic velocities increase slowly with depth from 4 9 km s l immediately below the superficial sediments to 6 3 km s l at the moho we interpret this latter structure as thinned continental crust surprisingly neither the seafloor topography nor the depth to moho reflect the transition from oceanic to continental crust we infer from these results that although the southern and eastern parts of the tyrrhenian basin were formed primarily by seafloor spreading much of the basin north of about 40 n and probably including some areas with water depths in excess of 3 km is underlain by thinned continental crust the evolution of the northern part of the tyrrhenian basin must therefore have occurred chiefly through a process of continental extension which in some areas has reduced crustal thickness to as little as 6 8 km south of 40 n

enigmatic fossils from the upper silurian of bolivia evidence for marine productivity in high latitude gondwana specimens collected recently from the las carmelitas facies of the kirusillas formation exposed in the cochabamba area bolivia and dated as ludlow late silurian by graptolites show bedding surfaces almost completely covered by coalified compressions and impressions the majority comprise mixtures of fragmented amorphous unidentifiable material debris but some horizons are dominated by well defined morphological entities with a particular type or combination of types confined to a particular layer in all five morphotypes have been circumscribed but their affinities based on a comprehensive review of the possibilities remain conjectural algal affinities are suggested tentatively for abundant unbranched narrow strap shaped fossils lacking any further diagnostic features and the numerous coalified discs or three dimensionally preserved spheres leiosphaerid acritarchs occurring isolated in the matrix or less often in clusters following a discussion on the recognition of faecal pellets in the fossil record it is postulated that elliptical spiral or simply segmented structures may be the excreta of planktonic or benthic metazoans although the latter seems less likely in view of the proposed anoxic depositional environment of the shales that lack any bioturbation such coalified remains whether from primary producers or grazers indicate high productivity at high palaeolatitudes in the late silurian continental seas in this region of peri gondwana

structural evolution of the tyrrhenian back arc basin abstract the tyrrhenian basin has developed as a marginal basin through drifting of the calabrian sicilian arc system towards the east southeast the main structural features of the basin result from this movement the tensional structures normal faults of the margins are mainly aligned n10 to n30 e the transform faults are oriented n110 n120 e except for a previous structuralization linked to the western mediterranean rifting and drifting the first extensional movements affected the tyrrhenian area as early as lower miocene time during the post burdigalian nevertheless the main rifting phase which led to oceanic accretion was initiated during the late tortonian the narrow central oceanic area generated mainly around the early pliocene is restricted to the rift s axis the marginal basin type of evolution of the tyrrhenian sea is most probably directly linked to subduction of the ionian lithosphere nevertheless interferences between collision in the surrounding apennines calabria or sicilides and subduction processes have dominated this special back arc basin evolution since late tortonian time the tyrrhenian arc shows a regional n120 e oriented tensional pattern short compressional events occur briefly in middle pliocene and middle pleistocene times correlated with brief paroxysmal phases of extension and subsidence the deformation depends on the presence of a subduction zone parallel to the calabrian arc swell which allows the migration of the unblocked segments of the arc in a n120 e direction

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

experimental evidence that an asteroid impact led to the extinction of many species 65 million years ago an overview is presented of the theory that an asteroid hit the earth 65 million years ago wiping out large numbers of land and marine species this theory predicts that the clay layer found at the boundary of the cretaceous and tertiary periods would be substantially different chemically from the clays in the limestone above and below the boundary layer it also predicts the iridium enhancement seen in this boundary layer would be found worldwide both predictions have been found to be true a third prediction was made but not fulfilled that the five major extinctions which occurred in the last 570 million years were caused by an asteroid impact the transport mechanism of the dust created by the impact and four mechanisms for the extinction of species are discussed criticisms of the theory are also discussed

structural pattern and co2 ch4 degassing of ustica island southern tyrrhenian basin brittle tectonics and ground degassing including fracture field soil gas and exhalation flux analyses of co2 and ch4 were studied at ustica island a pleistocene volcanic complex in the southern tyrrhenian sea the mesoscopic fracture pattern perfectly fits an e w trending left lateral strike slip master fault in agreement with the main morpho structural submarine alignment including ustica island and anchise seamount along the sw ne high angle normal arso fault geological evidence of reactivation with different kinematics left to right lateral displacements was recognised major co2 and ch4 degassing with fluxes up to 93 750 and 20 t km 2 a 1 respectively and soil gas concentrations of 11 3 and 5 7 ppm occur over the arso fault although this fault is mapped just in the sw sector of the island soil gas co2 anomalies point out its clear continuation up to the ne margin of the island these data together with those of previous geophysical and geochemical results from off shore ustica suggest that the arso fault is the local evidence of a more important active gas bearing structure this tectonic feature is interpreted as a reactivation of a preexistent sw ne trend inherited as a second order structure of the e w deep shear zone the reactivation is related to the interplay among different structures of the southern tyrrhenian basin

thermo mechanical modeling of the tyrrhenian sea lithospheric necking and kinematics of rifting we present the results of quantitative forward modeling of the sardinian rifted margin of the tyrrhenian sea the purpose of this study is to investigate the thermo mechanical structure that affects the thinning of the lithosphere across the margin the role of lithospheric necking during basin formation constrained by basement topography moho depth and gravity anomalies is modeled for different compensation models independent constraints are obtained from the analysis of the predicted thermal structure a deep level of necking 25 km is required to explain the observed crustal geometries and gravity anomaly signature the model predicts spatial and temporal variations in rheology during extension with important implications for kinematics of lithospheric thinning prerift lithospheric conditions strain rate and temperature during extension appear to be the key controls on the style of lithospheric rifting and necking in the tyrrhenian sea a forward model for basin stratigraphy is presented for the polyphase history of the tyrrhenian rifting this model provides quantitative estimates of time space dependent crustal thinning and rates of extension

jurassic extension estimates for the north sea triple junction from flexural backstripping implications for decompression melting models abstract the rattray volcanics lie at the triple junction between the three arms of the north sea rift system the mildly undersaturated volcanics in this province were erupted during the early syn rift phase of a middle late jurassic rifting event and are thought to be the products of decompression induced partial melting of the mantle resulting from lithospheric attenuation during rifting situated as they are in one of the most intensively studied rift basins in the world the rattray volcanics provide an excellent opportunity to evaluate the relationship between extension and melting described by mckenzie and bickle 1 this study uses a modified version of the backstripping technique which takes into account the flexural strength of the lithosphere to constrain the stretching factor for the jurassic rift phase two dimensional interpreted sections are flexurally backstripped to the base post rift to give an initial estimate of is then further constrained by forward modelling to the base post rift basin geometry generated by backstripping forward modelling uses the flexural cantilever model a combined simple shear pure shear model for continental extension which takes into account the thermal rheological and flexural isostatic consequences of extension this is an approach that is different from that of previous studies 2 5 which assume local rather than regional isostatic compensation to allow the backstripping of individual wells the results obtained in this new study give maximum jurassic factors on each of the three rift arms close to the triple junction of 1 2 1 35 this suggests a maximum jurassic factor in the triple junction itself of 1 35 1 75 this is considerably lower than previous estimates and is due to the breakdown of the assumption used in these previous studies that airy isostasy can approximate the response of a low flexural rigidity lithosphere in subsidence analysis whilst such a factor is insufficient to generate melt by decompression alone an anomalously high mantle potential temperature and the presence of volatiles as well as a complex mesozoic and late palaeozoic rifting history are likely to have played a role in the generation of melt

mineralogic evidence for an impact event at the cretaceous tertiary boundary a thin claystone layer found in nonmarine rocks at the palynological cretaceous tertiary boundary in eastern montana contains an anomalously high value of iridium the nonclay fraction is mostly quartz with minor feldspar and some of these grains display planar features these planar features are related to specific crystallographic directions in the quartz lattice the shocked quartz grains also exhibit asterism and have lowered refractive indices all these mineralogical features are characteristic of shock metamorphism and are compelling evidence that the shocked grains are the product of a high velocity impact between a large extraterrestrial body and the earth the shocked minerals represent silicic target material injected into the stratosphere by the impact of the projectile

thermal state and deep earthquakes in the southern tyrrhenian the geodynamic processes in the southern tyrrhenian are approached by comparing the thermal structure with the seismic activity subsidence data are used to determine the boundaries of the oceanic domains in the abyssal area the regional pattern of the surface heat flux reflects lateral variations of structural properties of the lithosphere in the oceanic domain a mean heat flux of 160 mw m 2 is observed to which a convective component of about 100 mw m 2 should be added if an ocean plate cooling model is applicable for a pure shear stretching model the heat flux values of the continental margins indicate a thinning factor of 3 5 4 0 the maximum frequency of seismic events occurs in the 200 350 km depth interval and the energy released is maximum between 250 and 300 km most earthquakes are confined within a 30 50 km thick zone the slab dip angle is about 65 in the 100 400 km depth range characterised by predominant down dip compression then both the slab and the maximum compression axis slightly deflect down to 500 km depth where the deepest event occurs under the hypothesis that the seismic activity is confined within portions of downgoing lithosphere which have a temperature below a depth dependent critical value calculations for a 60 70 km thick slab sinking at a velocity of 6 10 2 m yr 1 yield a thermal structure compatible with the distribution of the deepest seismic activity such a thermomechanical picture and the correlation between the slab dip angle and the total tectonic subsidence of the oceanic domain are typical features of a young marginal basin and are related to the drag stress in the vicinity of the downgoing lithosphere

metal precipitation in the cretaceous tertiary boundary clay at stevns klint denmark abstract alvarez et al 1980 explained anomalous ir concentrations in the cretaceous tertiary k t boundary informal type sediment the fish clay in denmark in terms of a global fall out of extraterrestrial matter however geochemical and mineralogical analyses of the various sublayers in the clay reveal that not only ir but also elements such as ni co zn mo as se and sb are substantially enriched in the clay the enrichments may be due to precipitation of elements as sulfides at a redoxcline the facts supporting this view are 1 1 the over all chalcophile element abundance pattern in the clay 2 2 the occurrence of a redoxcline deduced from the distribution of organic matter and oxidized iron in the clay 3 3 the large amounts of small pyrite spheroids present at this redoxcline 4 4 the high concentration of trace elements in these spheroids also ni co and ir elements that have previously been presumed to have a meteoritic origin are substantially enriched in the spheroids a model is suggested in which metal bearing pore solutions lose their metals by precipitation upon passing the boundary between the oxic cretaceous chalk and the overlying anoxic and hydrogen sulnde rich fish clay the great similarity in geochemistry and lithological appearance between several strongly ir enriched clays suggest that the model may also be applied to other k t boundary clays

vertical movements on the eastern margin of the tyrrhenian extensional basin new data from mt bulgheria southern apennines italy abstract in this paper we present the results of an integrated stratigraphical geomorphological and morphotectonic study carried out in mt bulgheria a rocky headland located on the eastern margin of the tyrrhenian extensional basin in southern italy such a margin is characterised by the occurrence of uplifted headlands alternated with subsiding half grabens in particular mt bulgheria constitutes the northeastern margin of the gulf of policastro peri tyrrhenian depression in the study area pliocene quaternary continental deposits and erosional landforms are present together with pleistocene wave cut and wave built marine terraces the integrated study of pliocene pleistocene deposits and landforms allowed a detailed reconstruction of the geomorphological evolution of mt bulgheria to be made furthermore the tectonic behaviour of this portion of the tyrrhenian margin during plio quaternary times could be characterised in particular we recognised that a succession of alternated uplift and subsidence episodes affected the headland since the beginning of early pleistocene when the eastward migrating coastline of the tyrrhenian extensional basin reached mt bulgheria the alternated uplift and subsidence events lasted for early and middle pleistocene their cessation in late pleistocene is demonstrated by the occurrence of late pleistocene strandlines at an elevation comparable with that of the 130 ka sea level highstand as recorded in stable areas of the mediterranean region the occurrence of marine terraces allowed the absolute vertical movements to be estimated their summation produced an uplift of the headland of 400 m since the beginning of the santernian 150 m of which occurred in the middle pleistocene along the onshore belt of the tyrrhenian margin of southern apennines other structural highs bordering the peri tyrrhenian depressions provide evidence for subsidence episodes interrupting the prevailing uplift trend recorded during the quaternary this together with the distribution of quaternary marine deposits both downthrown and raised and terraces shows that the tyrrhenian margin of southern apennines was affected by strongly variable vertical movements

exceptional preservation of fossils in an upper proterozoic shale late proterozoic organisms must have been diverse and widely distributed but in general their fossil record is both taxonomically and environmentally limited exceptional preservation of proterozoic fossils is not unknown but it is usually associated with silicified carbonates from restricted peritidal or playa lake environments1 2 we report here an exceptionally well preserved and distinctive assemblage of late proterozoic fossils from subtidal marine shales in addition to the sphaeromorphic acritarchs and cyanobacterial sheaths routinely preserved in proterozoic rocks this assemblage includes multicellular algae seaweeds a diverse assortment of morphologically complex protistan vesicles and probable heterotrophic bacteria thus it provides one of the clearest and most taxonomically varied views of proterozoic life yet reported

the ordovician sierras pampeanas puna basin connection basement thinning and basin formation in the proto andean back arc article i nfo the ordovician sierras pampeanas located in a continental back arc position at the proto andean margin of southwest gondwana experienced substantial mantle heat transfer during the ordovician famatina orogeny converting neoproterozoic and early cambrian metasediments to migmatites and granites the high grade metamorphic basement underwent intense extensional shearing during the early and middle ordovician contemporaneously up to 7000 m marine sediments were deposited in extensional back arc basins covering the pre ordovician basement extensional ordovician tectonics were more effective in mid and lower crustal migmatites than in higher levels of the crust at a depth of about 13 km the separating boundary between low strain solid upper and high strain lower migmatitic crust evolved to an intra crustal detachment the detachment zone varies in thickness but does not exceed about 500 m the formation of anatectic melt at the metamorphic peak and the resulting drop in shear strength initiated extensional tectonics which continued along localized ductile shear zones until the migmatitic crust cooled to amphibolite facies p t conditions p t d t data in combination with field evidence suggest significant ca 52 crustal thinning below the detachment corresponding to a thinning factor of 2 1 ductile thinning of the upper crust is estimated to be less than that of the lower crust and might range between 25 and 44 constituting total crustal thinning factors of 1 7 2 0 while the migmatites experienced retrograde decompression during the ordovician rocks along and above the detachment show isobaric cooling this suggests that the magnitude of upper crustal extension controls the amount of space created for sediments deposited at the surface upper crustal extension and thinning is compensated by newly deposited sediments maintaining constant pressure at detachment level thinning of the migmatitic lower crust is compensated by elevation of the crust mantle boundary the degree of mechanical coupling between migmatitic lower and solid upper crust across the detachment zone is the main factor controlling upper crustal extension basin formation and sediment thickness in the back arc basin the initiation of crustal extension in the back arc however crucially depends on the presence of anatectic melt in the middle and lower crust consumption of melt and cooling of the lower crust correlate with decreasing deposition rates in the sedimentary basins and decreasing rates of crustal extension

joint analysis of sar interferometry and electrical resistivity tomography surveys for investigating ground deformation the case study of satriano di lucania potenza italy abstract we have jointly applied microwave remote sensing imaging and ground based geophysical methodologies for investigating ground deformation in particular the differential interferometric synthetic aperture radar dinsar technique referred to as small baseline subset sbas algorithm and the electrical resistivity tomography ert method have been applied the former is a tool for investigating large areas allowing us to detect deformation phenomena and to analyse their spatial patterns and temporal evolution the latter permits us to reconstruct with high spatial resolution the geometry of near surface geological structures and to estimate the electrical resistivity of outcropping terrains the starting point of our analysis has been a dinsar survey carried out on an area of about 400 km2 centred around the town of potenza basilicata region italy for which sar data acquired by the ers 1 2 sensors in the time interval 1992 2000 were available the dinsar analysis allowed us to detect an unknown ground deformation effect which involves the urban area of satriano di lucania located close to potenza an on site ert survey was then carried out in order to characterize in detail the physical properties of the terrains involved in the main deformation pattern detected via the dinsar processing the integration of the dinsar and ert measurements permitted us to investigate areas where the deformation have space variant characteristics and allowed us to formulate a hypothesis on the origin of the detected displacement processes that although not fully conclusive is consistent with the overall dinsar and ert analysis

modes of stepwise eastward migration of the northern tyrrhenian sea back arc extension evidences from the northern latium offshore italy the structural architecture of a portion of the eastern side of the tyrrhenian sea back arc basin has been defined reinterpreting a public data set made up by seismic reflection profiles a detailed reconstruction of the extensional front migration trough time across the area has been achieved migration has been defined by identifying the generation of unconformities in the graben like basins developed in the area as due to the activation and deactivation of normal faults such phenomena are part of a tectonic process that also involved blocks rotation crustal thinning and stretching syn rift extension affected the area since the early to middle miocene our estimate principally interesting the north westernmost sectors during late miocene and throughout the plio quaternary the extensional front moved stepwise toward the southeastern and eastern sectors widespread postrift conditions established over the whole area since the late pliocene to early pleistocene since the quaternary the locus of active extension further migrated toward the present onshore area where a pronounced volcanic phase developed in this evolutionary framework the rates of observed migration could be correlated with the acceleration of calabrian slab rollback since pliocene furthermore direct evidences of magmatism were not recognized in the offshore although the observed crustal thinning the absence of volcanic bodies in the study area could be related to normal faults which were unable to sufficiently extend the crust in association with lower crust and mantle processes unable to produce melts

terminal cretaceous extinctions in the hell creek area montana compatible with catastrophic extinction inaccurate stratigraphic correlations in the hell creek area montana have led to the assumption that transitional vertebrate faunas bug creek anthills exist in the latest cretaceous refuting a catastrophic turnover at the cretaceous tertiary boundary establishment of the transitional faunas in paleocene channels that cut down through the cretaceous tertiary boundary renders the terrestrial faunal record compatible with the marine record and with catastrophic extinction

the earliest known palaeocene mammal fauna and its implications for the cretaceous tertiary transition several localities in northeastern montana usa representing the earliest known palaeocene mammal sites in the world document a gradual turnover in the mammalian fauna at the cretaceous tertiary boundary these localities in combination with previously known latest cretaceous localities represent the only well documented terrestrial sequence across the cretaceous tertiary boundary this sequence argues for gradual terrestrial extinctions and faunal changes at this boundary

prokaryotic and eukaryotic microfossils from a proterozoic phanerozoic transition in china cherts of the doushantuo formation in the yangtze gorges south china contain a superbly preserved heterogeneous assemblage of bacteria cyanobacteria planktonic algae submillimetre sized burrows and problematica the microbiota has a bearing on defining the precambrian cambrian boundary baltisphaeridium and associated spinose microfossils in the chert suggest an early cambrian age whereas other acritarchs vendotaenids and pretrilobite small shelly fossils from the conformably overlying the dengying formation indicate a late proterozoic age for the doushantuo this mixed benthic planktonic assemblage preserved in silicified laminated microbial mat like fabrics provides a unique sample of diverse microbiotic elements existing at the dawn of the phanerozoic and records the earliest evidence of meiofauna

sequence stratigraphy in proterozoic successions sedimentological logging and facies mapping have been used to identify depositional sequences bounded by subtle but regionally persistent unconformities in rocks of proterozoic age in the western united states south australia and northwestern canada we conclude from these studies that the sequence stratigraphic approach is of considerable importance for intrabasinal time correlation in the proterozoic and for facies interpretation and basin analysis in proterozoic rocks

a survey of paleozoic microbial fossils in chert abstract non acritarch microbial fossils from paleozoic marine cherts exhibit generally poor preservation with rare exceptional preservation microbial microfossils were discovered in 11 out of 17 stromatolitic and non stromatolitic paleozoic units containing replacement cherts from the continental united states many microfossils likely have a cyanobacterial affinity having both coccoid and filamentous morphotypes examined units range in age from middle cambrian to late permian and were deposited in peritidal open shelf and basinal settings all of which have preserved microbial assemblages cherts from the examined units generally formed during early diagenesis and the quality of microbial preservation does not seem to have changed during the paleozoic which is a period that experienced significant secular changes in the mode of marine silica formation

early neoproterozoic sequence stratigraphy and acritarch biostratigraphy eastern officer basin south australia abstract the early neoproterozoic torrensian latest tonian 800 750 ma succession in the central and eastern officer basin was deposited in an intra cratonic rift basin the outcropping pindyin sandstone consists of basal lowstand conglomerate and upward fining transgressive sandstone and is conformably overlain by marine highstand shale of the alinya formation in the giles 1 well the pindyin sandstone contains aeolian to fluvial deposits and is overlain by peritidal and sabkha sediments of the alinya formation seismic interpretation suggests that the succession is thin on the murnaroo platform and thickens toward the munyarai trough the two sedimentary provinces are separated by the ungoolya hinge which probably marks the southeastern margin of the fraser musgrave fold belt highstand organic rich shale in the alinya formation is probably an important petroleum source rock acritarchs occur in both chert and shale in the alinya formation and their distribution was probably controlled by sedimentary environments similar assemblages from outcrop and subsurface provide a reliable evidence for regional correlation the assemblages in giles 1 are dominated by benthic coccoids and contain many distinct spinose acritarchs which are particularly abundant in the subtidal grey shale whereas mainly phytoplankton is present in the chert the assemblage represented by trachyhystrichosphaera vidalii t aimica cymatiosphaeroides kullingii can be used for global correlation in the early neoproterozoic the abundant microfossils from the alinya formation and equivalent beds around the world probably indicate a microorganism explosion at 750 800 ma prior to the neoproterozoic glaciations thirty seven acritarchs spheroids and seven cyanobacterial filaments are briefly described in this study including 5 new species comasphaeridium tonium eoentophysalis gilesis goniosphaeridium alinyum gorgonisphaeridium pindyium and gorgonisphaeridium torrensium and a newly combined species archaeoellipsoides karatavicus comb nov

odp leg 107 in the tyrrhenian sea insights into passive margin and back arc basin evolution leg 107 of the ocean drilling program drilled a west northwest east southeast transect of seven sites across the tyrrhenian sea the youngest of the sub basins of the mediterranean sea sites 654 653 652 and 656 document the rifting and subsidence of the sardinia passive continental margin on the upper margin site 654 we cored a classic transgressive sequence subaerial conglomerates overlain by oyster bearing sands overlain by marine marl comparison between the recovered lithologies and seismic reflection profiles suggests that the synrift sediments on the upper margin are tortonian late miocene to messinian latest miocene in age whereas synrift sediments on the lower margin are messinian to pliocene in age during the messinian desiccation of the mediterranean sites 654 and 653 now on the upper sardinian margin apparently occupied a basinal setting where they received nannoplankton bearing clays interbedded with laminated gypsum sites 656 and 652 now on the lower sardinia margin were apparently higher standing during the desiccation event their messinian facies are subaerial and lacustrine respectively we infer from these lines of evidence that tilting and subsidence occurred more than a million years earlier on the upper margin than on the lower margin such diachroneity can be interpreted in terms of migration of the zone of maximum extension above a rolling back subduction zone or in terms of extension of continental crust by shear along a deep detachment fault sites 655 651 and 650 were drilled into two small basalt floored basins of the central and eastern tyrrhenian emplacement of basaltic crust in the central tyrrhenian vavilov basin apparently began more than a million years before emplacement of basaltic crust in the eastern tyrrhenian marsili basin this observation is compatible with previous suggestions that the tyrrhenian has grown southeastward in response to rollback of the down going slab that currently dips northwestward under the toe of italy at the easternmost site high vesicularity of the basalt and benthic foram assemblages in the oldest sediments imply that the basalt erupted in water shallower than 2 500 m it has apparently subsequently subsided to its present depth of 4 100 m below sea level nearly three times as fast as normal subsidence of crust formed at a mid ocean ridge

neogene and quaternary rollback evolution of the tyrrhenian sea the apennines and the sicilian maghrebides reconstruction of the evolution of the tyrrhenian sea shows that the major stage of rifting associated with the opening of this basin began at similar to10 ma it involved two episodes of back arc extension which were induced by the rollback of a west dipping subducting slab the first period of extension 10 6 ma was prominent in the northern tyrrhenian sea and in the western part of the southern tyrrhenian sea the second period of extension mainly affected the southern tyrrhenian sea began in the latest messinian 6 5 ma and has been accompanied by subduction rollback at rates of 60 100 km myr 1 slab reconstruction combined with paleomagnetic and paleogeographic constraints indicates that in the central apennines the latest messinian 6 5 ma arrival of a carbonate platform at the subduction zone impeded subduction and initiated a slab tear and major strike slip faults these processes resulted in the formation of a narrow subducting slab beneath the ionian sea that has undergone faster subduction rollback and induced extreme rates of back arc extension

problematic bedding plane markings from the middle proterozoic manganese subgroup bangemall basin western australia abstract enigmatic bedding plane markings impressions resembling isolated strings of beads occur in sandy and argillaceous strata of the stag arrow formation in the middle proterozoic manganese subgroup bangemall group of the bangemall basin east of newman western australia markings are abundant at two localities and traces are found at a third structures similar in both morphology and dimension have previously been reported from the appekunny formation in the middle proterozoic belt supergroup of u s a the manganese subgroup structures provide further details of morphology enabling comparison with a variety of biogenic and non biogenic structures a source for the markings still cannot be determined with certainty but a biogenic origin is favoured and affinities with the green or brown algae suggested such an origin is consistent with records of megascopic algae from rocks of similar age

multiple microtektite horizons in upper eocene marine sediments no evidence for mass extinctions microtektites have been recovered from three horizons in eight middle eocene to middle oligocene marine sediment sequences five of these occurrences are coeval and of latest eocene age 37 5 to 38 0 million years ago three are coeval and of early late eocene age 38 5 to 39 5 million years ago and three are of middle oligocene age 31 to 32 million years ago in addition rare probable microtektites have been found in sediments with ages of about 36 0 to 36 5 million years the microtektite horizon at 37 5 to 38 0 million years can be correlated with the north american tektite strewn field which has a fission track age minimum of 34 to 35 million years and a paleomagnetic age of 37 5 to 38 0 million years there is no evidence for mass faunal extinctions at any of the microtektite horizons many of the distinct faunal changes that occurred in the middle eocene to middle oligocene can be related to the formation of the antarctic ice sheet and the associated cooling phenomena and intensification of bottom currents that led to large scale dissolution of calcium carbonate and erosion which created areally extensive hiatuses in the deep sea sediment records the occurrence of microtektite horizons of several ages and the lack of evidence for faunal extinctions suggest that the effects of extraterrestrial bolide impacts may be unimportant in the biologic realm during middle eocene to middle oligocene time

patterns of phytoplankton radiation across the precambrian cambrian boundary late proterozoic and early phanerozoic phytoplankton comprise a wide array of microfossils of possibly bacterial and algal affinity their radiation patterns around the proterozoic cambrian boundary interval are becoming increasingly well known for numerous sequences in the east european and siberian platforms china baltoscandia and north america transgressive sequences in the east european platform and northern baltoscandia comprise alternating sandstones mudstones and claystones their sedimentology is well documented and microfossils acritarchs chitinous fossils sabelliditids shelly fossils and trace fossils are well represented rocks immediately overlying the transitional beds have yielded lower cambrian trilobite faunas that are utilized for regional correlations continuous successions of planktic microorganisms consisting of prokaryotic photoautotrophic cyanobacteria and algal protists are known from rocks comprising the transitional vendian cambrian sequence in the east european platform the rock sequence attributed to the lower cambrian platysolenites antiquissimus zone has yielded acritarchs of 9cambrian type9 abruptly replacing a morphologically simple proterozoic type microbiota recorded in beds of the upper proterozoic sabellidites vendotaenia zone the radiation of novel forms of phytoplankton appears to have been synchronous over a wide part of the late proterozoicarly cambrian shelf and is parallelled by the nearly contemporaneous emergence of cambrian shelly fossils and trace fossils along with new acritarch taxa the diagnostic acritarch assemblage of the platysolenites antiquissimus zone consists of widely distributed species occurring also in the lowermost cambrian deposits of the siberian platform baltoscandia greenland north america svalbard and scotland it is suggested that along with shelly faunas trace fossils and geochemical data the general radiation patterns of phytoplankton shed light on the events shaping the important biotic events around the precambrian cambrian boundary it is also believed that a better understanding of the regulatory mechanisms affecting the base of trophic chains may in turn be of significant importance in defining the precambrian cambrian boundary

clay mineralogy of the cretaceous tertiary boundary clay analyses of the clay mineralogy of samples from the cretaceous tertiary boundary layer at four localities show that the boundary clay is neither mineralogically exotic nor distinct from locally derived clays above and below the boundary the significant ejecta component in the clay that is predicted by the asteroid impact scenario was not detected

cenozoic denudation of corsica in response to ligurian and tyrrhenian extension results from apatite fission track thermochronology 1 the island of corsica france occupies a unique position in the western mediterranean since it has recorded both the cenozoic alpine orogenic history of the area as well as subsequent extensional collapse and oceanic basin formation we present 41 new apatite fission track aft ages and 23 measurements of track length distributions from corsica in order to elucidate its cenozoic thermal and morphological evolution aft ages vary from 10 5 0 8 ma to 53 8 4 1 ma and form a clear spatial pattern oldest ages are encountered in the southwest of the island with a broad band of ages between 20 and 30 ma running across the mountainous central area and ages 20 ma confined to the eastern half of the island samples along the western and northwestern coasts record kilometer scale erosional denudation linked to rifting in the ligurian provencal basin whereas samples from close to the extensionally inverted alpine deformation front record a later cooling phase related to tyrrhenian extension the eastward younging pattern of aft ages suggests the migration of a wave of erosional denudation from west to east across the island apparently controlled by the migrating locus of extension our aft data therefore support models of mediterranean extension controlled by slab roll back

latest proterozoic plankton from the amadeus basin in central australia the plankton of the proterozoic is preserved mainly as microfossils known as acritarchs most of which are considered to be cysts of eukaryotic algae in recent studies of the diversity of proterozoic and early palaeozoic acritarchs a gradual increase in diversity from the middle into the late proterozoic has been shown to be followed by a sharp decrease in diversity in the latest proterozoic ediacarian or vendian and then a rise again in the early cambrian1 2 these observations have been interpreted in terms of the evolutionary diversification of eukaryotic algae and the ecological effects of the pre ediacarian glaciation occasional discoveries of complex acritarchs in ediacarian rocks3 6 have raised some doubts about these interpretations but more recent work seems to have supported this view7 here we report the discovery of a diverse assemblage of large and morphologically complex acritarchs from the ediacarian age upper pertatataka formation in the amadeus basin of central australia this with a recent report of similar fossils from the upper sinian of china8 provides a significant new perspective on the history of the plankton it is now necessary to suggest an earlier radiation at least in off shore environments or to question the reality of the postulated decrease in diversity in addition there may well have been an extinction event late in the ediacarian neither of these phenomena has been recognized previously

styles of back arc extension in the central mediterranean in the central mediterranean two back arc basins the liguro provencal lpb and the tyrrhenian basin tb opened progressively and consecutively from the late eocene oligocene to the present evolution in space and time of rifting and drifting processes along three different transects across these basins shows differences in the style of extension lpb opened with the formation of a narrow single rift while in the tb deformation and magmatism is spread over a wide area moreover at the northern end of the tb the locus of extension progressively migrated towards the east whereas in the southern tb the locus of extension and magmatism migrated inside the basin inducing continental break up and drifting of the previously formed older conjugate basins we propose that these different styles of back arc extension depend upon internal conditions such as prerift rheology linked with its geological heritage and external conditions e g the style of subduction

thermal evolution of a rift basin the tyrrhenian sea existing geological and geophysical data support the view that the opening of the tyrrhenian sea was by rifting of a formerly continuous continental lithosphere which began in upper miocene on the upper sardinian margin and migrated southeastward to the calabria margin heat flow across the tyrrhenian sea shows a pronounced asymmetry from about 50 70 mw m 2 over the sardinian margin to more than 150 mw m 2 over its southeastern margin off southern italy and calabria in this study we use the newly acquired data from ocean drilling program leg 107 heat flow seismic velocity and bathymetric data to constrain a group of models that supposes the tyrrhenian sea opened by detachment of a formerly continuous lithosphere along a southeastern facing low angle normal fault specific models tested include 1 the wernicke model in which thinning and extension of the continental lithosphere are supposed to be accomplished by simple shear along a large scale gently dipping detachment fault zone which cut through the entire lithosphere and 2 the delamination model in which the low angle detachment fault is supposed to cut only the upper and middle crust but to merge subhorizontally into the lower crust below which concurrent pure shear may occur a versatile two dimensional finite element procedure is used to evaluate the tectonic and thermal evolution of these models and the results are compared with observations to test the validity of the models delamination models with large pure shear components in the lower lithosphere and the wernicke model predict heat flow within 1 of the measurements the wernicke model however predicts a basin configuration much too deep in comparison with bathymetry and a subbasinal lithospheric thickness substantially different from rayleigh wave dispersion results the delamination model with a large component of pure shear in the subbasinal lower lithosphere on the other hand predicts satisfactorily both the bathymetry and the lithospheric thickness beneath the basin

architecture and neogene to recent evolution of the western calabrian continental margin an upper plate perspective to the ionian subduction system central mediterranean the western calabria continental margin forms the transition between the pliocene to recent marsili spreading center and continental calabria all parts of the upper plate of the ionian subduction zone integrating high resolution and crustal seismic images constrained by gravity modeling we provide a detailed reconstruction of the architecture of the margin and develop a new scheme for its miocene to present evolution this time span encompasses the continent continent collision between africa and eurasia subsequent orogenic collapse and rifting apart between the two continental masses and the pliocene to recent emplacement of oceanic crust in the vavilov and marsili basins the crust of the margin thins from the calabria coast 25 km to the marsili continent ocean transition 12 km on the whole upper and lower crusts thin proportionally with pure shear geometry the continental margin is covered by an oligocene to present sedimentary succession reaching a maximum thickness of 6 0 km in the paola basin during the miocene the continental margin experienced regional shortening accommodated by a large number of mainly west vergent thrusts possibly associated with the late stages of the kabilo calabrian chain shortening continued through pliocene to recent but was accommodated by a limited number of west vergent thrust faults located in the western part of the margin and by a few tens of kilometers wide syncline located in the eastern part of the profile the accommodation space created in the syncline core hosted a 4 5 km thick plio quaternary sedimentary succession the paola basin no significant extensional fault is observed along the profile miocene to recent subsidence was controlled by 1 a short wavelength component related to shortening and responsible for the formation of the paola basin syncline and possibly contributing to the uplift of onshore calabria and 2 a long wavelength component responsible for the regional subsidence and oceanward tilting of the calabria margin short wavelength subsidence ended in the late pliocene but long wavelength downward movements persisted and even accelerated during late pliocene to quaternary times when the present day bathymetry was achieved both horizontal deformations and vertical movements are difficult to explain in the context of a normal back arc basin without taking into consideration patterns of secondary mantle flow generated by the retreat of the subducting slab