seasonal variability in the production of lower cretaceous calcareous nannoplankton the lower cretaceous munk marl bed in the central north sea area consists of alternating laminae of calcareous nannofossils and clayey material the lamination was most likely caused by seasonal variation in the production rate of calcareous nannofossils calcareous nannofossil deposition reflects short blooms many blooms were totally dominated by a single species and at least nine species occasionally monopolized the flora the production rate and mode of deposition is comparable to that of the modern black sea the munk marl bed contains a diverse marine calcareous nannofossil flora comprising 48 species the species composition exhibits changes that show similarities to the seasonal fluctuations in the modern atlantic ocean

late cretaceous cenozoic tectonic development of the southwest pacific region abstract a new model of the plate tectonic development of the southwest pacific integrates the continental geology of new zealand with the age structure of the surrounding oceanic crust revealed previously from magnetic anomaly lineations the model differs from previous ones in that the onland geology of new zealand is used to constrain the tectonic development in two important ways 1 the modern australia pacific plate boundary did not transect the new zealand sector until 23 million years ago 2 there has been a total of only c 500 km of dextral displacement on the plate boundary through new zealand the model is described with reference to a series of paleotectonic maps drawn to represent the setting at the times of anomalies 32 24 7 and 5 novel interpretations based on these reconstructions include the following during the late cretaceous the campbell fault was a continental transform fault between the tasman sea spreading centre and the bounty rift its 330 km of dextral displacement accompanying 25 of counterclockwise rotation of a campbell plateau block when the eastward propagating southeast indian ridge broke into the south tasman sea 57 million years ago it continued to propagate directly into southern south island and generated late eocene oligocene continental rifting through western new zealand at the same time back arc spreading in the norfolk basin caused continental rifting along a similar trend in western north island the australia pacific plate boundary originated as a transform fault between the southeast indian ridge and the pre existing kermadec trench it developed in response to the sudden cessation of spreading on the segment of the southeast indian ridge east of a fracture zone here named fracture zone z two implications arise from this model regarding the integrity of antarctica 1 1 during the cretaceous marie byrd land was probably no more than 200 km northwest of its present position 2 2 the proposition made in some earlier plate tectonic reconstructions that if the alpine fault did not form until after the late eocene there must be a late cretaceous early tertiary plate boundary within antarctica is most probably valid

sr isotope stratigraphy assigning time in the campanian pliensbachian toarcian and valanginian abstractthe trend of marine 87sr 86sr against stratigraphic level through sections whether linear or not can identify hiatuses and changing rates of sedimentation through those sections and so be a valuable constraint on attempts to assign numerical ages to sediments on the basis of astrochronology or u pb dating of zircons here we illustrate that value for the campanian pliensbachian toarcian and valanginian ages by comparing 87sr 86sr profiles for different localities and comparing those to the 87sr 86sr profile through time the analysis reveals possible problems both with current time scales and with some astrochronological calibrations our analysis is neither comprehensive nor final rather with a few examples we show how sr isotope stratigraphy can be used to moderate other methods of assigning numerical ages to sediments

long period seismic radiation from the may 23 1989 macquarie ridge earthquake evidence for coseismic slip in the mantle long period seismic source parameters of the may 23 1989 macquarie ridge earthquake are determined using a surface wave inversion procedure that incorporates detailed source time functions obtained from shorter period body waves the seismic source model obtained using this method is consistent with observations of both body waves and long period rayleigh waves from the earthquake the macquarie ridge earthquake rupture has a centroid time 28 s and a right lateral strike slip fault mechanism with a rake of 175 on a vertical fault plane striking n38 e this mechanism is consistent with p wave first motions of the event inversions performed for various earth models demonstrate that the choice of surface wave attenuation model in particular affects the estimates of centroid depth and seismic moment significantly allowing for uncertainty in attenuation the long period rayleigh waves periods from 150 to 300 s indicate that the macquarie ridge earthquake had a seismic moment of 1 9 0 2 1021 n m and a corresponding centroid depth of 21 6 km the static stress drop calculated using the depth and seismic moment is 0 7 0 5 107 pa 70 50 bars the centroid depth combined with the lack of resolvable directivity of the earthquake rupture suggests that significant slip occurred beneath the moho which has a maximum depth of about 16 km in the epicentral region the mantle slip component may have preferentially radiated long period seismic energy given the shallow centroid depths and low moments determined for the event from the shorter period body wave observations

intraplate earthquakes in the southwest pacific ocean basin and the seismotectonics of the southern tasman sea an examination of 311 intraplate earthquakes in the australian plate portion of the pacific ocean basin reported from 1918 to 1990 reveals that only 113 events are reliably intraplate with most of the rest relocating to active trenches and transforms the non random distribution of the reliably intraplate events gives insight into the tectonic stresses present the central tasman sea is mostly aseismic except for a swarm of activity at the predicted site of the tasmantid hot spot to the north the broad regions of the coral sea south fiji basin and lord howe rise show very little intraplate seismicity yet the narrow norfolk ridge and three kings rise caught between the double convergence of the new hebrides and tonga subduction zones support many more earthquakes high levels of intraplate seismicity in the southern tasman sea adjacent to the macquarie ridge complex mrc indicate that this region may be undergoing internal deformation due to the unusual nature of the australia pacific plate boundary additional support exists in the form of intraplate focal mechanisms similar to those at the plate boundary and a set of parallel gravity rolls which are observed in recent geoid maps some aftershocks of the mw 8 2 macquarie ridge earthquake of 1989 occurred in a fracture zone west of the macquarie ridge complex das 1992 but we have found several earthquakes from as early as 1924 which relocate to this feature suggesting that its reactivation may be more significant than previously thought this reactivation of a fossil fracture zone may be the result of the increasing amount of oblique convergence between the australia and pacific plates at the macquarie ridge complex formerly a spreading center and the stresses associated with subducting recently formed australian ocean crust beneath the older pacific plate

sea level reconstructions from the peri vocontian zone south east france point to valanginian glacio eustasy the valanginian is a period of global environmental change as illustrated by sedimentary palaeontological geochemical and climatic perturbations a production crisis in most of the carbonate platforms suggests important changes in palaeoenvironmental conditions during the same time interval a major positive excursion in 13c the weissert event suggests perturbations of the carbon cycle from the latest early valanginian to the early hauterivian in order to better understand the link between these changes sea level fluctuations have been reconstructed in detail from the middle berriasian to the earliest hauterivian sections from the peri vocontian zone south east france have been investigated because of the good quality of outcrops on the carbonate platforms their margins and in the vocontian basin sections ranging from the most proximal zone swiss jura to the basin were interpreted in terms of sequence stratigraphy and cyclostratigraphy and correlated at high resolutions using the identified small medium and large scale sequences as well as depositional geometries sea level fluctuations were reconstructed two main trends are evidenced during the studied interval i the peak amplitude magnitude of the sea level fluctuations increased gradually from the middle berriasian to the early valanginian and reached a maximum more than 50 m from the middle early valanginian to the valanginian hauterivian boundary and ii sea level variations were quite symmetrical during the late berriasian slightly asymmetrical during the early valanginian and strongly asymmetrical fast sea level rise slow fall from the latest early valanginian to the earliest hauterivian moreover three orders of sea level fluctuations were recognized in the sedimentary rocks of the peri vocontian zone platform basin correlations and cyclostratigraphic interpretations of the basinal sections evidence an astronomical control on the sea level variations mainly by the two eccentricity cycles of 100 and 400 kyr the increase in the amplitude of the sea level fluctuations and their change from symmetrical to asymmetrical can be related to the onset of a major cooling event in the early valanginian fast transgressions followed by slower regressions would correspond to waxing and waning of high latitudinal ice during most of the valanginian especially from the latest early valanginian to the latest late valanginian glacio eustatic sea level fluctuations in tune with the 100 and 400 kyr eccentricity cycles are in agreement with glaciations during the valanginian

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

ammonoid and nannoplankton abundance in valanginian early cretaceous limestone marl successions from the southeast france basin carbonate dilution or productivity a quantification of calcareous nannofossil and ammonoid absolute abundance was carried out in the valanginian limestone marl alternations of the vergol section drome located in the epicontinental vocontian basin southeast france this section is characterised by four centimetric organic carbon rich layers barrande layers b1 b4 the aim of this work is to reconstruct changes in the distribution of nutrients in the water column on the base of the analysis of fossil organisms at different levels of the food chain calcareous nannofossil abundance correlates negatively with carbonate content thus calcareous nannoplankton probably did not produce the bulk of carbonates in the valanginian of the studied section nannofossil abundance was more likely controlled by carbonate mud produced and exported from shallow platform environments to the basin variations of carbonate export controlled by climate and hydrodynamics are responsible for the limestone marl alternations the levels of highest abundance of nannoplankton are interpreted as mainly linked to a reduced accumulation rate and to a minor extent to variations in primary production a dilution by allochthonous carbonate mud is further evidenced by the relationship between total organic carbon toc and calcium carbonate caco3 toc content decreases with increasing caco3 content except for one of the organic carbon rich layers b4 both taphonomy and dilution factors may have partly controlled the variations in macrofauna abundance these can be more likely interpreted as a response to palaeoenvironmental changes of the various taxa with different palaeoecological affinities the high abundance and dominance of bochianitids orthocone ammonoids in the layer b4 can be explained by their opportunistic behaviour and their deep nektonic possibly nektobenthic mode of life their great capacity for vertical mobility may have permitted brief feeding incursions to the bottom waters when the trophic resources were higher in depth than in the surface waters high trophic levels at the sea bottom may have also benefited the bivalves and gastropods which are very abundant in layer b4 the mode of life of different ammonoid families is also discussed

folding of oceanic lithosphere folding of the lithosphere just south of the bay of bengal appears as 1 undulations in acoustic basement topography and 2 as linear geoid undulations in the seasat altimeter data from the seasat data we find that the east west trending folds have wavelengths ranging from 130 to 250 km and clustering about 190 km the horizontal gravity disturbances due to the folds range in amplitude from 15 to 50 mgal elastic models of oceanic lithosphere have in the past been used to demonstrate the implausibility of lithosphere buckling or folding in response to compression these elastic models typically predict that compressive stresses of about 5 gpa are required to buckle oceanic lithosphere with an age comparable to that of the northeastern indian ocean 40 70 ma these stresses exceed the strength of lithospheric rock we use an elastic plastic model to show that oceanic lithosphere of this age should have a net compressive strength equal to about 12 of the elastic buckling stress we further demonstrate that loads approaching the net compressive strength cause the lithosphere to fold with a wavelength about 200 km i e the wavelength observed from seasat our results reinforce earlier speculation that this folding may be related to the himalayan orogeny

tertiary carbonate dissolution cycles on the sierra leone rise eastern equatorial atlantic ocean abstract most of the tertiary section on sierra leone rise off northwest africa consists of chalk marl and limestone that show cyclic alterations of clay rich and clay poor beds about 20 60 cm thick on the basis of biostratigraphic accumulation rates the cycles in oligocene and miocene chalk have periods which average about 44 000 years and those in eocene siliceous limestone have periods of 4000 27 000 years several sections were sampled in detail to further define the cycles in terms of content of caco 3 clay minerals and relative abundances of calcareous nannofossils extending information gained by analyses of pleistocene cores from the continental margin of northwest africa to the tertiary cycles on sierra leone rise both dilution by noncarbonate material and dissolution of caco 3 could have contributed to the observed relative variations in clay and caco 3 however dissolution of caco 3 as the main cause of the carbonate clay cycles on the sierra leone rise rather than dilution by clay is suggested by the large amount of change several thousand percent in terrigenous influx required to produce the observed variations in amount of clay and by the marked increase in abundance of dissolution resistant discoasters relative to more easily dissolved coccoliths in low carbonate parts of cycles the main cause of dissolution of caco 3 was shoaling of the carbonate compensation depth ccd during the early neogene and climatically induced fluctuations in the thickness of antarctic bottom water

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

intraplate deformation and closure of the australia antarctica africa plate circuit to determine the current motion between the australian antarctic and african plates and to test whether this plate circuit obeys closure all plate motion data available along the southeast southwest and central indian ridges are analyzed and reduced to 67 spreading rates 38 transform fault azimuths and 135 earthquake slip vectors carlsberg ridge data were excluded because they record india africa not australia africa motion new data include 10 new transform azimuths along the southwest indian ridge many slip vectors and a dense aeromagnetic survey along the southeast indian ridge all published and many unpublished magnetic profiles are modeled to determine rates consistently over a 3 m y time averaging interval for three reasons 1 magnetic profiles from the southeast and central indian ridges suggest recent spreading rate changes 2 some published rates differ for identical profiles and 3 prior studies have not used identical criteria and time scales to determine spreading rates the new rates differ from published rates by as much as 5 mm yr transform fault azimuths are estimated from bathymetry seasat altimetric data epicenter distributions and offsets of magnetic lineations azimuths of plate motion are also estimated from the horizontal projections of slip vectors from the centroid moment tensor solutions and other focal mechanism studies our new australia antarctica and australia africa euler vectors differ from all prior euler vectors at the 95 confidence level along the southeast and central indian ridges our model gives rates differing by 4 7 mm yr from those of prior models from a systematic analysis of the plate motion data we find no evidence for a nubia somalia antarctica triple junction along the southwest indian ridge we also find no evidence for a triple junction previously proposed to be at 80 e along the southeast indian ridge any deformation within this seismically active region of the australian plate adds up to less than a few millimeters per year along each boundary data are fit well by a single euler vector except for a 5 systematic misfit to azimuthal data along the eastern southeast indian ridge near a seismically active region of the australian plate south of tasmania although seismicity and the azimuthal misfit suggest the australian plate is deforming measurably possibly by distributed deformation or by the westward motion of a small microplate southeast of tasmania the significance of the misfit is marginal and is small enough that systematic errors may cause it these alternative explanations could be distinguished by surveying these transform faults with modern seafloor mapping techniques prior studies found that enforcing plate circuit closure causes systematic misfits to rates along the southeast indian ridge here we find that enforcing closure causes no systematic misfits to any data moreover only insignificant nonclosure is found by an f ratio test which numerical experiments suggest could detect deformation exceeding 2 mm yr we conclude that indian ocean plate circuit nonclosure and the deformation it suggests are much smaller than thought before the absence of significant nonclosure argues against the usefulness of a model of deformation distributed throughout an indo australian plate but favors a model in which the significant deformation occurs in a diffuse plate boundary along the equatorial indian ocean between the central indian ridge and the sumatra trench

astrochronology of the valanginian stage from reference sections vocontian basin france and palaeoenvironmental implications for the weissert event abstract high resolution gamma ray measurements performed on five biostratigraphically well dated reference sections from the vocontian basin south eastern france are used to develop a new astrochronology of the valanginian stage and its subdivisions i e ammonite and calcareous nannofossil zones and subzones spectral analyses show a pervasive dominance of 405 kyr eccentricity cycles with the expression of 100 kyr eccentricity obliquity and precession previous rough estimates of valanginian stage duration ranged from 3 9 to 6 5 myr but were generally based on less reliable or indirect methods this study provides a precise duration of 5 08 myr tuning the series to the 405 kyr eccentricity cycle thus further enhancing the accuracy of the geologic time scale new durations of magnetochrons are proposed based on intercalibration between ammonite and calcareous nannofossil biozones cyclostratigraphy and carbon isotope stratigraphy after integrating orbitally calibrated durations and available radio isotopic ages for the berriasian hauterivian interval a link between the weissert event and the emplacement of the parana etendeka traps now appears unlikely strontium carbon and oxygen isotope stratigraphy sedimentological and palaeontological features clay mineralogy and the orbital record during the upper valanginian all suggest that a transient cool arid climate occurred after the carbon isotope event possibly associated with limited polar ice

astronomical frequencies for pre quaternary palaeoclimate studies the sensitivity of the earth s orbital frequencies involved in the astronomical theory of palaeoclimates to the earth moon distance and consequently to the length of the day and to the dynamical ellipticity of the earth has been investigated for the last billion years the shortening of the earth moon distance and of the length of the day back in time induces a shortening of the fundamental astronomical periods for the obliquity and the precession a shortening which is larger for the longer periods these periods of precession and obliquity influenced by the changing earth moon distance length of the day and dynamical ellipticity of the earth have been recomputed for the last 440 million years

quantitative variations in the valanginian ammonite fauna of the vocontian basin southeastern france within limestone marl cycles and within parasequence sets abstract the ammonite assemblages between beds and interbeds are examined this quantitative analysis was carried out on limestone marl alternations in the valanginian vergol section drome located in the vocontian basin southeast france there is a strong faunal contrast between beds and interbeds the calcareous beds are characterized by high relative abundances of neocomitids and olcostephanids whereas the marls are marked by a high relative frequency of phylloceratids significant modifications of the composition of ammonite spectra are also observed from the base to the top of some parasequence sets the variations in bathymetry and trophic drifts linked to the climatic fluctuations may have controlled the ammonite assemblages the increase in bathymetry in marls was thus probably beneficial to phylloceratids which are a deep water group the high planktonic production in limestones could explain the great relative abundance of neocomitids and olcostephanids which lived in the upper part of the water column

intraplate seismicity of the pacific basin 1913 1988 we establish here a comprehensive database of intraplate seismicity in the pacific basin relocation and analysis of 894 earthquakes yield 403 reliable intraplate earthquakes during 1913 1988 these numbers do not include earthquake swarms which account for another 838 events most of the remainder 304 events are actually plate boundary earthquakes that have been erroneously located in intraplate regions a significant number occur in recent years when location capabilities should have guarded against this situation relocations involve a careful linear inversion of p and s arrivals accompanied by a monte carlo statistical analysis we have also attentively removed the high number of clerical errors and nuclear tests that exist in epicenter bulletins

extraction of high resolution carbonate data for palaeoclimate reconstruction temporal variations in the calcium carbonate content of deepsea sediments provide direct stratigraphic as well as important palaeoenvironmental information relating to the global carbon cycle here i present an algorithm that allows carbonate content and porosity to be accurately predicted from saturated bulk density in equatorial pelagic carbonates applying the algorithm to continuous laboratory measurements of density made on dsdp and oop cores yields a nearly continuous carbonate record for the upper 200 m of the sediment section long ultra high resolution carbonate curves of this type should yield new insight into the evolution of the carbon chemistry of the oceans as well as the role of external milankovitch forcing in the development of the carbonate system the algorithm can also be applied to quantitative high resolution seismic data thereby enabling detailed carbonate records to be extracted from remotely derived geophysical data

the valanginian isotope event a complex suite of palaeoenvironmental perturbations abstract the valanginian records a severe crisis of carbonate systems both on platforms and in the pelagic realm this crisis is roughly concomitant with the weissert event characterized by a positive 13c excursion of about 2 in marine carbonates however it is unclear if the response of these two carbonate systems to the global perturbations is contemporaneous or if they react differently for this purpose accumulation rates of pelagic carbonates produced by nannofossils and of platform derived carbonates have been quantified in a hemipelagic environment the vocontian basin se france that has the potential to record the reaction of both shallow water and pelagic carbonate production to a major perturbation of the carbon cycle testified by the isotope excursion also changes through time of clay input have been measured in order to evaluate variations in continental weathering rates and thus in continent derived nutrient input and climatic conditions in terms of humidity aridity accumulation rates of pelagic carbonates of platform derived carbonates and of clays were calculated on the basis of nannofossil absolute abundance and species specific masses of weight percent carbonate content of rocks and a high resolution timescale given by cyclostratigraphy the onset of the positive 13c excursion and the demise of the platforms testify for a synchronous but inverse response of the shallow water and pelagic carbonate systems to an increase in nutrient influx to marine waters as carbonate production by nannofossils decreased during the late valanginian in time of still low platform production the low temperature of marine waters coeval to a still high nutrient input recorded in the latest late valanginian possibly could explain that both platform and pelagic carbonate production contemporaneously decreased

a rare great earthquake on an oceanic fossil fracture zone first motion solution found in this study and has a strike consistent with the fracture zone north of the bend the southern fault plane with strike 178 d ip 54 and rake 65 has a strike consistent with the portion of the fracture zone to the south of the bend and has a dip which can be explained by the style of deformation that the region is undergoing the centroid moment tensor solution of the broad band model is calculated and found to be consistent with the region of low misfit in mantle wave solution space the broad band solution has a moment of 1 53 10 21 nm mw 8 1 again consistent with the mantle wave data slip propagated bilaterally with an approximate rupture velocity of 3k ms 1 80 per cent of local shear wave speed the rupture front is less well resolved to the south of the epicentre than to the north the majority 75 per cent of moment originated from slip on the northern fault the broad band data requires significant slip below the oceanic moho with as much as 70 per cent of moment due to slip in the brittle uppermost mantle in the preferred model

climate and sea level variations along the northwestern tethyan margin during the valanginian c isotope excursion mineralogical evidence from the vocontian basin se france a high resolution mineralogical study bulk rock and clay fraction was carried out upon the hemipelagic strata of the angles section vocontian basin se france in which the valanginian positive c isotope excursion occurs to investigate sea level fluctuations and climate change respectively a detrital index di phyllosilicates and quartz calcite and a weathering index wi kaolinite illite chlorite were established and compared to second order sea level fluctuations in addition the mineralogical data were compared with the high nutrient index hni based on calcareous nannofossil taxa data obtained by duchamp alphonse et al 2007 in order to assess the link between the hydrolysis conditions recorded on the surrounding continents and the trophic conditions inferred for the vocontian basin it appears that the mineralogical distribution along the northwestern tethyan margin is mainly influenced by sea level changes during the early valanginian pertransiens to stephanophorus ammonite zones and by climate variations from the late early valanginian to the base of the hauterivian top of the stephanophorus to the radiatus ammonite zones the sea level fall observed in the pertransiens ammonite zone early valanginian is well expressed by an increase in detrital inputs an increase in the di associated with a more proximal source and a shallower marine environment whereas the sea level rise recorded in the stephanophorus ammonite zone corresponds to a decrease in detrital influx a decrease in the di as the source becomes more distal and the environment deeper interpretation of both di and wi indicates that the positive c isotope excursion top of the stephanophorus to the verrucosum ammonite zones is associated with an increase of detrital inputs under a stable warm and humid climate probably related to greenhouse conditions the strongest hydrolysis conditions being reached at the maximum of the positive c isotope excursion from the verrucosum ammonite zone to the base of the hauterivian radiatus ammonite zone climatic conditions evolved from weak hydrolysis conditions and most likely a cooler climate resulting in a decrease in detrital inputs to a seasonal climate in which more humid seasons alternated with more arid ones the comparison of the wi to the hni shows that the nutrification recorded al the angles section from the top of the stephanophorus to the radiatus ammonite zones including the positive c isotope shift is associated with climatic changes in the source areas at that time increased nutrient inputs were generally triggered by increased weathering processes in the source areas due to acceleration in the hydrological cycle under greenhouse conditions this scenario accords with the widely questioned palaeoenvironmental model proposed by lini et al 1992 and suggests that increasing greenhouse conditions are the main factor that drove the palaeoenvironmental changes observed in the hemipelagic realm of the vocontian basin during the valanginian positive c isotope shift this high resolution mineralogical study highlights short term climatic changes during the valanginian probably associated to rapid changes in the c cycle coeval massive parana etendeka flood basalt eruptions may explain such rapid perturbations c 2011 elsevier b v all rights reserved

the kinematic evolution of the macquarie plate a case study for the fragmentation of oceanic lithosphere abstract the tectonic evolution of the southeast indian ridge seir and in particular of its easternmost edge has not been constrained by high resolution shipboard data and therefore the kinematic details of its behavior are uncertain using new shipboard magnetic data obtained by r vib araon and m v l astrolabe along the easternmost seir and available archived magnetic data we estimated the finite rotation parameters of the macquarie antarctic and australian antarctic motions for eight anomalies 1o 2 2ay 2ao 3y 3o 3ay and 3ao these new finite rotations indicate that the macquarie plate since its creation 6 24 million years ago behaved as an independent and rigid plate confirming previous estimates the change in the australian antarctic spreading direction from n s to nw se appears to coincide with the formation of the macquarie plate at 6 24 ma analysis of the estimated plate motions indicates that the initiation and growth stages of the macquarie plate resemble the kinematic evolution of other microplates and continental breakup whereby a rapid acceleration in angular velocity took place after its initial formation followed by a slow decay suggesting that a decrease in the resistive strength force might have played a significant role in the kinematic evolution of the microplate the motions of the macquarie plate during its growth stages may have been further enhanced by the increased subducting rates along the hjort trench while the macquarie plate has exhibited constant growth by seafloor spreading

signal analysis of cyclicity in maastrichtian pelagic chalks from the danish north sea abstract low field bulk magnetic susceptibility has been determined on maastrichtian chalk samples from a drill core from the dan field in the danish north sea fast fourier transformations fft have been used to detect possible cycles in the magnetic susceptibility data power spectra from the complete section and from sub sections of the magnetic susceptibility reveal two cyclicities of ca 0 4 cycles m and ca 1 7 cycles m which are present on a 90 confidence level signal analysis of natural gamma ray wire line log data supports these findings sedimentation rate estimates place the cycles in the milankovitch frequency band artificial time series are used to study the applicability of the fft to identifying cyclicity in chalks expected geological distortions e g hiati and sedimentation rate variations are introduced into the time series to investigate the response of the frequency spectra different methods of handling missing data intervals are also examined

multistratigraphic records of the lower cretaceous valanginian cenomanian puez key area in n italy abstract we present a stratigraphic investigation of a hauterivian to cenomanian hemipelagic succession from col puez in the dolomites southern alps northern italy a composite section of seven segments was studied with bio magneto and chemostratigraphy which enabled detailed and robust age calibration of the puez succession it also revealed the paleoceanographic history and helped identify the important global climatic events of that period the age of the lower cretaceous puez formation in the dolomites is refined as late hauterivian to early cenomanian ammonoids provide a detailed biostratigraphic subdivision that forms the basis for analyses of the faunal distribution and the paleoenvironmental evolution of basins and plateaus across the tethys the succession consists of facies similar to those of coeval units in the tethys apart from the late barremian to late aptian interval which is marked by a major hiatus followed by a few meters thick redbed of late aptian age the lower cretaceous was characterized by a weak 1 positive 13c trend punctuated by a positive shift in the late hauterivian corresponding to the faraoni level oxygen isotope values show a decreasing trend from the hauterivian to the albian cenomanian which mirrors a long term increasing trend in paleotemperatures from 15 18 c to 25 30 c this is in agreement with global paleotemperature reconstructions for this period representing the onset of mid cretaceous super greenhouse conditions nonetheless no black shale occurs at the intervals equivalent to aptian albian oceanic anoxic events oae suggesting unique paleoceanographic conditions enhanced current activity on a submarine height on the trento plateau area during the mid cretaceous

a quantitative study of lower cretaceous cyclic sequences from the atlantic ocean and the vocontian basin se france milankovitch type periodicity was identified within lower cretaceous cyclic sequences from the western central atlantic ocean and the vocontian basin southeast france through application of the walsh spectral method to detailed stratigraphic sections the repetitious bedding in these sequences appears to be the product of cyclic climatic changes controlled by the orbital forcing obliquity seems to be the most important and persistent orbital element responsible for cyclic sedimentation whereas precession is less important upper berriasian and valanginian strata from the atlantic ocean contain indications of eccentricity induced periodicity and of a nonlinear response of the climatic system variations in orbitally tuned pelagic sedimentation rates in the atlantic ocean correlate with the changes in the character of cycles and with the occurrence of calcareous turbidites therefore this suite of sedimentary features are responding to evolving paleoceanographic conditions and possible relative sea level fluctuations the orbitally tuned durations of the valanginian and hauterivian stages in the vocontian basin are a minimum of 5 9 and 5 3 m y respectively

the 1992 landers earthquake sequence earthquake occurrence and structural heterogeneities the june 28 1992 mw 7 3 landers earthquake occurred in the southeastern mojave desert california over 10 000 aftershocks of the earthquake were recorded by the caltech usgs southern california seismic network scsn in 1992 to investigate the relationship between complexities in the crustal structure and variations in seismicity we have used a tomographic method to invert 145 098 p wave arrival times from 3740 landers earthquake aftershocks and 1148 other events recorded by 60 permanent and temporary scsn stations we determined a detailed p wave tomographic image with a spatial resolution of about 5 km and relocated the hypocenters with the obtained 3 d velocity model the results show a correlation between seismicity patterns and velocity patterns and a tendency for regions rich in seismicity to be associated with higher velocities the higher velocity areas are considered to be strong and brittle parts of the fault zone which are apt to generate earthquakes in contrast low velocity areas are probably more ductile and weaker allowing aseismic slippage

milankovitch climatic origin of mid cretaceous black shale rhythms in central italy the earth s orbital variations reflected in pleistocene ice volume were also recorded in non glacial times carbonate production in pelagic mid cretaceous sediments quantified by calcium carbonate and optical densitometry time series reflects the orbital eccentricity and precessional cycles minimal eccentricity brought deep sea anoxia most of the sedimentary variability of this 100 myr old sequence lies within the milankovitch orbital frequency band

reactivation of an oceanic fracture by the macquarie ridge earthquake of 1989 theoretical calculations show1 that the occurrence of an earthquake leads to regions of increased shear stress on both sides of the ruptured fault if this stress increase is sufficiently large or if neighbouring faults are close to their failure stress the result may be triggering of earthquakes on pre existing faults in the region a search for this effect2 produced only a few examples all in continental areas where clusters of aftershocks were located in the regions of increased stress the linear dimensions of these clusters were generally less than 20 km and less than one half of the fault length that ruptured in the main shock here i report an example of this phenomenon in which the fault that is reactivated is a nearly 175 km long segment of an oceanic fracture and its length is comparable to that of the main shock the 1989 macquarie ridge earthquake

climatic precession is the main driver of early cretaceous sedimentation in the vocontian basin france evidence from the valanginian orpierre succession the valanginian sediments outcropping in the vocontian basin se france exhibit striking marl limestone alternations which were formed under the influence of orbital forcing and which have served for geochronological and paleoenvironmental studies previous studies have suggested an obliquity forcing during the late valanginian interval reflecting specific environmental conditions such as polar ice using a cyclostratigraphic correlation of previously studied sections and performing time series analysis on the most complete late valanginian interval we argue that the climatic precession cycle is the primary driver of these marl limestone alternations in addition we highlight the modulation of the precession by the 100 and 405 kyr eccentricity cycles we suggest that the cyclostratigraphic misinterpretation i e obliquity forcing hypothesis results mainly from poorly preserved 405 kyr eccentricity cycles due to local hiatuses and or missed beats this study shows the potential of cyclostratigraphic correlations for the detection and quantification of differential hiatuses and or missed beats within intrabasinal sequences hence providing constraints on cyclostratigraphic interpretations the recorded 405 kyr eccentricity cycle is of prominent amplitude and controlled the fourth order sea level sequences these latter are faithfully detected through cyclostratigraphically inferred sedimentation rate finally we show that the well known pronounced lithostratigraphic markers intervals in the basin were orbitally paced by the 405 kyr eccentricity extrema this is a good argument for the strong impact of this cyclicity on the sedimentary processes especially during greenhouse periods

the 1987 1992 gulf of alaska earthquakes abstract we present a study of the 1987 1992 gulf of alaska earthquake sequence using relocated seismicity data together with body wave analysis of selected larger events most of the sequence is located on a n s trending fault directly south of the yakataga seismic gap and consists of four main events of mw 7 2 7 8 7 7 and 6 8 and associated aftershocks the first earthquake is of left lateral strike slip type on an ene wsw trending fault the second event is a right lateral strike slip earthquake on the n s trending fault the fault plane of the third strike slip event was identified as an ene wsw trending fault located to the south of that for the first event in a previous study using body wave analysis we show that though the body wave study cannot unambiguously identify the fault plane the temporal development of the seismicity together with the pattern of aftershock distribution on the conjugate fault suggests that this event also occurred on the n s trending fault and is of right lateral type the seismicity on the conjugate fault is interpreted as being triggered by the increase of the shear stress in the direction of the normal to the fault plane due to the main shock the occurrence of the fourth main shock a right lateral strike slip event in 1992 which itself can be considered an aftershock of the 1987 1988 sequence with epicentres distributed along a n s trending fault favours this conclusion the first three events of the sequence have been described in earlier studies as having too short a rupture length for their seismic moment if the rupture lengths inferred from the aftershock zones in this study are used we find that this is not the case the events in the 1987 1992 sequence lie along magnetic anomaly 13 and related conjugate fracture zones indicating that the oceanic crust is rupturing along pre existing zones of weakness in response to plate boundary stresses at the southern end the seismic activity terminates near a seamount which rises 1 km above an otherwise relatively low relief ocean floor and close to a fracture zone perpendicular to the n s trend of the earthquake sequence suggesting that the n s striking rupture along the n s trending fault is terminated here by a barrier at the northern end the sequence terminates at the alaska trench near the yakataga seismic gap where there is a right lateral offset in the bathymetry along the transition fault this offset also being visible on the gravity map of the region thus this offset has the same right lateral sense as the n s trending strike slip fault planes of three of the main shocks and suggests that though there was no significant seismic activity in this region of the gulf of alaska since 1964 faulting here has been active in the past

what caused the march 25 1998 antarctic plate earthquake inferences from regional stress and strain rate fields we investigate possible driving forces behind the occurrence of the 1998 antarctic plate earthquake we determine first a regional strain rate field associated with the accommodation of relative plate motion and second a vertically averaged minimum deviatoric stress field associated with lithospheric gravitational potential energy differences and deglaciation of the antarctic ice cap we find that the mechanism of this event is inconsistent with strain orientations inferred from kinematic modeling of a diffuse zone of deformation within the triple junction region stress perturbations associated with deglaciation cannot be ruled out as a triggering mechanism for this event

tectonics and distribution of gold deposits in china an overview gold exploration in china has expanded rapidly during the last two decades since a modern approach to economic development has become a national priority china currently produces 180 tonnes t of gold annually which is still significantly less than south africa usa and australia however china is now recognized as possessing significant gold resources in a wide range of mineral deposit types present estimates of gold resources in china exceed 4 500 t which comprise 60 in gold only deposits more than 25 in base metal rich skarn porphyry and vein deposits and more than 10 in placer accumulations the major gold provinces in china formed during the main episodes of phanerozoic tectonism such tectonism involved interaction of china s three major precambrian cratons north china tarim and yangtze or south china when combined with cathaysia block with the angara or siberian kazakhstan kyrgyzstan and indian cratons resulting collisions included deformation of accreted oceanic sequences between the cratonic blocks the most important ore forming orogenies were 1 the late paleozoic variscan 405 270 ma which led to amalgamation of the angara north china and yangtze cratons 2 the indosinian 270 208 ma which led to the collision of north china and south china cratons 3 the yanshanian 208 90 ma which was largely influenced by the subduction of the izanagi pacific plates beneath eastern china and 4 the himalayan 90 ma indentation of the indian continent into eurasia no important precambrian gold systems are recognized in china mainly because of reworking of exposed precambrian rocks by these younger orogenies but there are a few caledonian 600 405 ma gold bearing systems in northern xinjiang most of china s orogenic epithermal and carlin like gold deposits are in the reworked margins of major cratonic blocks and in metasedimentary rock dominated fold belts adjacent to these margins accordingly the major gold provinces are present along the northern southeastern and southern margins of the north china craton along the southwestern and northwestern margins of the yangtze craton in the tianshan and altayshan orogenic belts in northern xinjiang and throughout the southeastern china fold belt gold placer deposits derived from these primary deposits are concentrated in the northernmost part of northeastern china and along the northwestern margin of the yangtze craton the major provinces with significant gold in porphyry related copper systems and base metal skarns are present in the yangtze river area along the northeastern and southeastern margin of the yangtze craton in the fold belt in southwestern china and scattered through northern china three quarters of the chinese gold only deposits occur within the north china craton margins half are located in the uplifted precambrian metamorphic rocks and most of the remainder are hosted in the phanerozoic granitoids that intruded the reworked precambrian terranes the abundance of granite hosted gold contrasts the north china craton with other precambrian cratons such as those in western australia central canada and zimbabwe where gold is mainly hosted in the archean greenstone belts this difference may be explained by the multiple episodes of phanerozoic tectonism along the north china craton margins resulting from the collision of the angara north china and south china cratons and from subduction of the izanagi pacific oceanic plates underneath the eastern china continent

depositional sequences in shallow carbonate dominated sedimentary systems concepts for a high resolution analysis abstract oxfordian and berriasian sections representing shallow water carbonate dominated sedimentary systems have been studied in the swiss and french jura in spain and in normandy they all display a hierarchical stacking of depositional sequences facies evolution and stacking pattern allow to define elementary small scale medium scale and large scale sequences some depositional sequences display well marked sequence boundaries others are limited by transgressive or maximum flooding surfaces the hierarchical organisation of such sequence stratigraphic elements implies that sea level fluctuations were an important factor in their formation and that these fluctuations had different frequencies the superposition of high frequency sea level changes on a long term sea level trend leads to repetition of diagnostic surfaces defining sequence boundary and maximum flooding zones wherein the corresponding high frequency surfaces are well developed chronostratigraphic tie points permit us to estimate the duration of large scale sequences this time control and the observed hierarchical stacking suggest that the high frequency sea level changes were controlled by climatic cycles in the milankovitch frequency band the variability of stacking pattern and facies evolution between sections illustrates the complexity of the studied environments furthermore because of the minimal accommodation space available in these shallow water settings much of the geologic time is not recorded nevertheless detailed analysis of the depositional sequences allows the interpretation of the evolution of the sedimentary system with a high time resolution thus there is a potential to monitor sedimentological ecological and diagenetic processes on a time scale of 20 to 100 ka

tracking high frequency seismic source evolution 2004 mw 8 1 macquarie event the 2004 mw 8 1 event on 23 december 2004 near the macquarie ridge is a very large intraplate event that has been overshadowed by the mw 9 3 sumatra andaman event only 3 days later we are able to track the progress of source evolution by estimating the progression of the points of energy emission exploiting the good azimuthal distribution of available stations the results indicate that this event ruptured on two nearby fault systems reactivating fossil fracture zones with the second subevent to the west triggered by the first the total duration of high frequency radiation is quite short about 60 s for such a large event much of the high frequency radiation occurs on a fault subparallel to that inferred from long period studies this composite fault behavior with displaced triggered failure appears to be a characteristic of large intraplate events beneath the oceans

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

spectral analysis of the middle triassic latemar limestone abstract the middle triassic latemar limestone in the dolomites region northern italy consists of hundreds of vertically stacked meter scale platform carbonate cycles each cycle is composed of a shallow marine subtidal carbonate unit with a thin centimeter scale vadose diagenetic dolomite cap these latemar couplets are the products of repeated episodes of submergence emergence with durations estimated at 20 000 yr the couplets are arranged in upward thinning bundles of five indicating the presence of a 100 000 year modulating component we examined this high frequency cycling in the platform buildup using a variety of classical and advanced spectral analysis techniques we found si nificant evidence for the presence of a milankovitch forcing signal in this cycling we also present evidence that the adjacent coeval basinal facies the livinallongo formation exhibits a similar bundling pattern in its cm scale hemipelagic carbonate shale alternations we conclude that milankovitch forced small amplitude sea level oscillations were the most likely cause of these middle triassic carbonate rhythms

a warm equable cretaceous the nature of the problem abstract the nature of the problem of warm equable paleoclimates is defined by investigating the mid cretaceous period the problem consists of three components 1 defining precisely the climatic state during any specified interval of geologic time 2 specifying the external climatic forcing factors which may have been important and 3 understanding the climatic response to any specific modifying influence these components are characterized by limitations of critical importance in understanding paleoclimates the nature of the problem of a warm cretaceous is defined both qualitatively and quantitatively within these limits

kinematic evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we present an updated series of kinematic reconstructions of the major plates around the tethys from the atlantic ocean to the pamirs between the early jurassic and the present this set is used elsewhere as a basis for paleogeographic maps of the entire region the problems related to the positions of the continents in the lower triassic are also discussed no direct analyses of magnetic anomalies and fracture zones in the atlantic have been made rather all available poles and rotations have been tested in order to eliminate or minimize possible kinematic errors the reconstructions are shown for nine key geological periods which correspond to well recognized magnetic anomalies except for the jurassic cretaceous boundary and the cretaceous tertiary boundary which correspond to interpolated positions paleolatitudes have been drawn using the study of westphal et al 1986 an attempt has been made to take into account the displacements caused by formation of the continental margins and basins by stretching the resulting relative vector of motions along the northern boundary of the tethys shows a significant change 80 m y ago left lateral motion with compression dominates before whereas right lateral motion with compression dominates after to the east rates of motion vary by a factor of three with time and four maxima can be clearly related to tectonic events in the late jurassic late cretaceous eocene and post middle miocene to the west north of apulia on the contrary the motion rate has not changed significantly since the early cretaceous and is close to 1 cm yr as an average these rather complex adjustments in rates and directions of relative motion are produced in great part through a complex migration of the africa eurasia pole of rotation and seem to be mostly governed by the tectonics of the tethys plate boundary

the detection and illustration of regular sedimentary cycles using walsh power spectra and filtering with examples from the lias of switzerland power spectral analysis provides an objective method for the detection of regular cyclicity in data recording some oscillating parameter or time series digitization of measured stratigraphic sections is a convenient method for generating long time series during the examination of pelagic sequences for cyclicity related to orbital variations or milankovitch cycles the application of walsh power spectral analysis to stratigraphic sections is outlined and band pass filtering using the walsh transform introduced it is shown that variations in sedimentation rate hiatuses bioturbation and variations in compaction have the effect of damping or destroying spectral peaks the wavelength ratios of spectral peaks are often used during studies of milankovitch cycles to decide which orbital cycles are involved however in some circumstances hiatuses are able to alter wavelength ratios averaged walsh power spectra were generated for three liassic limestone shale sequences from breggia gorge switzerland the 21 000 year 21 ka precession cycle was detected in all three cases the 100 ka eccentricity cycle in two cases and the 41 ka obliquity cycle in one filtering revealed that the supposed 21 ka sedimentary cycles are grouped into packets of four or five small and large amplitude cycles the packets reflect the 100 ka modulation of the 21 ka precession cycle local disruption of the packaging can be used to locate hiatuses in pelagic strata

fertilization of the northwestern tethys vocontian basin se france during the valanginian carbon isotope perturbation evidence from calcareous nannofossils and trace element data a high resolution calcareous nannofossil study associated with a geochemical analysis major trace elements and carbon and oxygen isotope stratigraphies was carried out in the angles section hemipelagic setting of the vocontian basin se france during the valanginian positive carbon isotope excursion the behaviour of calcareous nannofossil taxa in relation to fertility conditions was studied to elaborate new nutrient indices in this environment a high nutrient index based on biscutum spp discorhabdus rotatorius zeugrhabdotus fissus high fertility indicators and watznaueria barnesae low fertility indicator and a medium nutrient index based on lithraphidites carniolensis medium fertility indicator and w barnesae low fertility indicator these two indices show a major fertilization from the stephanophorus ammonite zone to the trinodosum ammonite zone with a maximum during the positive carbon isotope excursion since high values of the nutrient indices are in phase with high values of chemical elements related to terrigenous material and low values of the coccolith total abundance it is proposed that pulses of detrital inputs into the basin triggered the nutrification which in turn caused a biocalcification crisis of the calcareous nannofossils nutrification is also responsible for the reef demise in the surrounded platforms as indicated by the increased sr ca seawater ratio at that time the intensification of the parana etendeka volcanic activity triggering co2 excess in the atmosphere is probably responsible for an acceleration of the hydrological cycle the increased weathering and the subsequent higher terrigenous and nutrient transfer from continents to the vocontian basin in such a scenario nutrification is a dominant factor controlling neritic and hemipelagic biocalcification however one cannot exclude that the global increase of atmospheric co2 could generate chemical changes of the sea surface waters acting with the nutrification to modify the biocalcification of the carbonate producers

controls on cyclostratigraphy of lower cretaceous carbonates and evaporites cupido and coahuila platforms northeastern mexico the lower cretaceous cupido barremian aptian and coahuila albian carbonate platforms of northeastern mexico exhibit thick successions of meter scale cycles deposited in three unique paleoenvironmental settings 1 the cupido shelf lagoon is composed of peritidal carbonate cycles deposited in the protected lee of a reef rimmed to barrier shoal margin 2 the restricted coahuila ramp interior consists of cyclic alternations of subtidal evaporites and peritidal carbonates 3 the deep water parts of both the cupido and coahuila platforms are composed of foraminiferal wackestones and lime mudstones interspersed with firmgrounds and hardgrounds in a cyclic arrangement vertical successions of meter scale evaporitic cycles and peritidal cycles exhibit systematic stacking patterns that build into intermediate scale high frequency sequences tens to hundreds of meters thick and large scale composite sequences hundreds of meters thick that can be correlated across the cupido and coahuila platforms these large scale stacking patterns are interpreted to reflect longterm accommodation events and when combined with the scale independent architecture of all genetic units permit the inference that all three meter scale cycle types on the cupido and coahuila platforms are also governed by relative sea level change the composition thickness and number of meter scale cycles within individual high frequency sequences can be highly variable across the cupido and coahuila platforms however even though the overall upward shallowing patterns are evident the lateral complexity of cycle architecture and distribution is interpreted to be a natural response to fluctuations in regional climate interacting with autogenic processes such as variations in carbonate production and dispersal intensity and frequency of tropical storms and monsoons thermohaline circulation patterns and ambient ocean chemistry and temperature these interacting processes created laterally variable physiographic and oceanographic conditions across the cupido and coahuila platforms complicating the sedimentary record generated by the composite sea level signal well documented evidence from barremian cenomanian pelagic cycles throughout the tethyan seaway strongly indicates that milankovitch driven climatic changes operated during the early cretaceous contemporaneous shallow marine cyclicity in several locations suggests that these climatic changes may have had globally widespread effects in an effort to link the shallow water and deep water realms we propose a model whereby milankovitch driven global climatic changes generated low amplitude high frequency eustatic fluctuations through some combination of thermal expansion and contraction of ocean water waxing and waning of small ice caps and alpine glaciers and changes in the storage capacity of aquifers and lakes to produce meter scale cycles across lower cretaceous shallow marine platforms

long climatic time series from sediment physical property measurements abstract with an accurate understanding of the lithologic dependence of pre compaction porosity in pelagic sediments it should be possible to calculate mass accumulation rate histories of sedimentary phases to generate proxy carbonate logs from core density scans and from seismic profiles and to reconstruct the mechanical and chemical processes that occur during burial and lithification we demonstrate that routinely made continuous gamma ray attenuation porosity evaluator grape wet bulk density measurements of sediments show a positive relationship to carbonate content in a variety of pelagic environments mineralogically related differences in packing rather than differences in grain density appear to control wet bulk density variations a hyperbolic model calibrated at dsdp odp drill ites quantifies the relationship of density to carbonate content in different pelagic environments the curvilinear relationships appear to converge to a similar wet bulk density 1 71 0 07 g cm3 porosity 58 at 100 carbonate content but they exhibit differing porosity at zero carbonate content wet bulk densities for a given carbonate content depend on the non carbonate mineralogy and grain size and increase systematically from opal to clay to quartz lithic fragment dominated settings because contrasts in sediment wet bulk density control the seismic response of unlithified sediments it should be possible to generate carbonate profiles remotely in carbonate rich settings after core calibration of the seismic system our model can generate long proxy carbonate curves with a typical sample spacing of 0 6 1 cm from existing grape data bases in equatorial pacific equatorial atlantic and northern atlantic locations over the past 6 m y these time series are used to investigate the long term evolution of climate in the milankovitch frequency band as well as to reconstruct patterns of carbonate sedimentation over time sedimentation bears the imprint of milankovitch cyclicity superimposed on long term trends the switch in carbonate sedimentation from 41 kyr periodicity during the matuyama chron to dominant 100 kyr periodicity in brunhes chron previously described in the north atlantic occurs in equatorial atlantic and pacific locations as well the lithological dependency of porosity in uncompacted pelagic sediments also has important implications for understanding later compaction and diagenetic modification of pelagic sediments in particular the grape observations suggest that differential burrow flattening observed in lithified limestone marl alternations results from mechanical and pressure solution compaction of strata that had strong primary porosity differences rather than from diagenetic unmixing of carbonate

constraints on macquarie ridge tectonics provided by harvard focal mechanisms and teleseismic earthquake locations in this study we evaluate teleseismically determined focal mechanisms and epicenters for earthquakes along the macquarie ridge complex mrc from 45os to 61 os and 155oe to 168oe a region characterized by some previous investigators as undergoing subduction initiation from 65 centroid moment tensors reported by harvard we develop statistical guidelines for choosing 26 which represent better determined more reliable focal mechanisms for tectonic analysis although thrust mechanisms occur in the north near fiordland elsewhere along the mrc the better determined mechanisms virtually all indicate that present day motion along most of the mrc is strike slip this is consistent with sidescan sonar and multichannel reflection data collected between 50os and 57os on 1994 and 1996 cruises the active plate boundar3 zone appears to be quite narrow 5 km wide and no active compressional features can be observed on the seafloor if we determine a rotation pole for plate boundary motion using only slip vectors from better determined harvard mechanisms along the mrc the best fitting instantaneous pole is at 57 4os 179 4oe about 2 5 o north of the nuvel i australian pacific pole which averages motion over the last 3 0 my if the mrc pole was formerly farther south than at present this could explain the existence of relict features associated with crustal shortening such as bathymetric highs and troughs yet the absence of active features such as thrust faults etc suggests ongoing compression or subduction initiation we also carefully read arrival times for p phases for 53 earthquakes at 16 teleseismic stations selected to represent a range of azimuths surrounding the earthquakes we relocated these earthquakes using standard joint epicentral determination jed methods while most of the better quality relocations lie on or very close to the australia pacific boundar3 as determined on the 1994 cruise a few epicenters occur well away from the boundar3 apparently on cenozoic fracture zones thus on the macquarie ridge complex and other major strike slip boundaries e g in california it appears that the very largest earthquakes occur on the principal plate boundar3 fault but that other earthquakes some quite large may occur away from the boundary along zones of preexisting weakness