mesozoic and cenozoic reconstructions of the south atlantic abstract the movement of antarctica with respect to south america has a number of implications for paleocirculation as well as for the reconstructions of gondwanaland recent papers on the southwest indian ridge have published new or revised poles of opening for africa and antarctica which can be combined with the poles of opening between south america and africa to give resultant motions between south america and antarctica the first indication of a complete closure between south america and the antarctic peninsula is at anomaly 28 time 64 ma as the two continents are now configured between anomaly 28 time 64 ma and anomaly m0 time 119 ma the amount of closure does not change greatly and the small computed overlap can be explained by minor uncertainties in the rotation poles used for the reconstructions or some slight extension between east and west antarctica by 135 ma some rotation or translation of the antarctic peninsula with respect to east antarctica must be postulated in addition to any presumed extension between east and west antarctica in order to avoid an overlap of south america with the antarctic peninsula having determined what we feel to be a viable reconstruction of western gondwanaland and holding south america fixed we rotated africa and antarctica with respect to south america for eight different times during the past africa moved away from south america in a more or less consistent manner throughout the time period closure to present while antarctica moved away from africa in a consistent manner only between 160 ma and 64 ma at 64 ma its motion changed abruptly it slowed its north south motion with respect to africa and began slow east west extension with respect to south america this change supports the hypothesis that a major reorganization of the triple junction between africa antarctica and south america occurred between 60 and 65 ma the triple junction changed from ridge ridge ridge to ridge fault fault at the time of the major westward jump of the mid atlantic ridge just south of the falkland agulhas fracture zone the mesozoic opening of the somali basin moved madagascar from its presumed original position with africa in gondwanaland the closure of sri lanka with india produces a unique fit for india and sri lanka with respect to africa madagascar and antarctica this fit juxtaposes geological localities in southeast india against similar localities in enderhy land east antarctica the late jurassic opening in the somali basin is tied to opening of the same age in the mozambique basin since this late jurassic movement represents the initial break up of gondwanaland it is assumed that similar movement must have occurred in what is now the western weddell sea and may also explain the opening evidenced by the rocas verdes region of southern south america

isostasy of the southern mid atlantic ridge long wavelength and short wavelength effects the isostatic transfer function for the southern mid atlantic ridge was computed from 25 profiles of gravity and topography normal to the ridge trend as follows from the comparison with previous studies the transfer function for the mid atlantic ridge is insensitive to the tectonic setting of profiles used the experimental transfer function was employed to remove all coherent short wavelength features in the gravity and topography from 17 long profiles across the southern mid atlantic ridge the profiles were then recomputed as a function of t age of the lithosphere and averaged over equal ages the observed regional topography regional gravity anomaly and altimeter geoid anomaly agree well with conductive cooling models of the spreading lithosphere an observed slope of the topography versus t1 2 and of the geoid versus t can be explained with a single set of model parameters comparison with the estimates by haxby for the northern mid atlantic and reykjanes ridges suggests a systematic decrease in the temperature of the mantle from 1500 1600 c near iceland to 1000 1100 c at the southern mid atlantic ridge the theoretical gravity anomaly deduced by the analytical method of haxby and turcotte is sensitive to the thermal parameters only in the age range 0 20 m y the fit of the theoretical to the observed gravity anomaly is good outside the axial strip 0 3 m y the poor fit at the axis is attributed to inadequacy in the boundary condition of the model

normal faulting and the topographic roughness of mid ocean ridge flanks the ocean floor on the flanks of mid ocean ridges is covered by abyssal hills topographic features elongated perpendicularly to the direction of relative plate motion these topographic features are interpreted as normal fault blocks and or volcanic constructions that originated near the ridge axis and were later rafted onto the ridge flanks by seafloor spreading the purpose of this paper is to quantify the topographic roughness of a profile perpendicular to the strike of a number of normal faults given a fault population and a mechanical model for the response of the lithosphere to faulting we obtain expressions for the variation in root mean square roughness with profile length and for the power spectral density of a profile given three parameters a fault density number of faults per unit length crossed by the profile an average squared fault scarp height and a characteristic length of flexure to keep matters simple we make a number of assumptions and approximations namely that the lithosphere behaves as an elastic plate that faults have an infinite length and a vertical dip that the response of topography to a number of faults is simply the sum of the responses to each fault and that faults have random locations and scarp heights independently chosen from some statistical distribution the theory predicts that the roughness length relationship power spectral density should follow power laws for scales wavelengths less than a characteristic scale proportional to the length scale of flexure we compare the predictions of the theory with actual measurements of mid ocean ridge flank roughness and find good first order agreement in particular we use independent estimates of fault densities average squared fault scarp heights and flexural length scales to predict the topographic roughness of the east pacific rise and we find that normal faulting can explain all the observed topographic roughness nevertheless there are some differences between predictions and observations these differences are likely to be due to processes other than faulting that create topographic relief e g volcanism and to spatial correlations of fault scarp heights despite these shortcomings the approach presented here provides a first step in understanding the topographic roughness signal by quantifying the contribution of the geological processes that generate surface relief

isostatic response of the alula fartak and owen transforms in the eastern gulf of aden and the adjoining arabian sea summary the alula fartak and owen transforms are the active parts of major fracture zones with a distinct topographic expression in the eastern gulf of aden and the arabian sea respectively while the alula fartak transform offsets the sheba ridge by about 180 km and is associated with a broad steep sided valley with a relief of nearly 3 5 km the owen transform offsets the carlsberg ridge by nearly 300 km and is associated with a broad step like valley surrounded by deeper water depths the gravity and topography data along several profiles selected across these two transforms have been analysed using cross spectral analysis in order to investigate their isostatic compensation the observed admittance estimates have been compared with three theoretical isostatic compensation models two local compensation models airy i and ii and one regional compensation plate model comparing the longer wavelength admittance estimates suggests that the regional compensation model gives the best fit for both the alula fartak transform and the owen transform with effective elastic thickness te of 5 km and slightly less than 10 km respectively for the alula fartak transform the airy ii model might also be acceptable though with large scatter in the observed values it suggests a mean value of 9 km for the mantle layer with a 6 km thick crust for the owen transform on the other hand the two local compensation models failed the difference in te estimate between the two transforms could be ascribed to differences in thermal structure arising from their varied tectonic history a comparison with the isostatic response estimates of transform fracture zones along the slow spreading mid atlantic ridge suggests that the regional compensation model is generally applicable for transform fracture zone topography along such mid ocean ridges

isostasy of fracture zones in the atlantic ocean the cross spectral method of studying the isostasy was applied to profiles of gravity and topography which run across fracture zones in the atlantic ocean between 40 sup 0 n and 40 sup 0 s 30 section in total were used each section 400 km long in wavelengths 25 to 80 km the coherence of the free air gravity anomaly and topography attains 0 8 while the phase is close to zero which indicates that the disturbances in gravity are controlled by the topography and its isostatic compensation the experimental isostatic response function admittance deduced in this study is compared with previous results which relied on different spatial distribution of data a single mechanism of isostasy can explain well the gravity anomalies across fraction zones and across the median valley of the mid atlantic ridge a basic feature of this mechanism is the elastic lithosphere 6 to 9 km in thickness we show that the topography and gravity anomalies over the mid atlantic ridge crest are essentially three dimensional nevertheless the admittance is reliable computed from a set of profiles by using the method of d mckenzie and c bowin this possibility is provided by the favorable pattern of the topography spectrum in the area under consideration

carlsberg ridge northern indian ocean gravity and isostasy summary gravity and bathymetry profiles across the carlsberg ridge in the indian ocean are analysed to investigate the isostatic compensation of this part of the indian ocean spreading ridge system the ridge has a rough flanking topography and a median valley which are the characteristics of slow spreading ridges we use the crossspectral technique to obtain an admittance function for isostatic studies admittance estimates in the long wavelength region 125 500 km indicate that the elastic plate thickness supporting the long wavelength topography is in the range 5 10 km we have also examined the isostasy of the axial region synthetic topography and gravity profiles computed for a cooling plate model are subtracted from the observed to estimate the median valley signatures the residual gravity anomaly over the ridge axis is mostly explained by the median valkey topography with a uniform crustal thickness of 6 km indicating that the median valley is not in isostatic equilibrium

lithospheric flexure at fracture zones bathymetric profiles across six major fracture zones fz s in the north pacific are used to demonstrate the absence of vertical slip on the fossil fault planes the scarp heights on these fz s are constant with age and equal to the initial vertical offsets at the ridge transform fault fz intersections because of the frozen in scarp and the differential subsidence of lithosphere far from the fz the lithosphere bends in the vicinity of the fz this flexure results in a characteristic ridge trough topographic fz signature the flexural amplitude which is the difference between the scarp height and the overall change in depth across the fz increases with age good fits to the bathymetric profiles across the mendocino and pioneer fz s are obtained by modelling the topography as the flexure of a thin elastic plate with an age dependent effective elastic thickness results of the modelling indicate that the base of the elastic lithosphere is approximately defined by the 450 c isotherm maximum bending stresses at fz s are on the order of 100 mpa substantially less than the stresses encountered at subduction zones because the mendocino and pioneer fz s are separated by less than a flexural wavelength they are elastically coupled

on isostasy at atlantic type continental margins cross spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern north america southwest africa and the coral sea lord howe rise the resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy the relatively young coral sea lord howe rise margin is associated with the lowest value of the effective elastic thickness te 5 km while the relatively old eastern north american margin is associated with the highest value 10 te 20 km thus te appears to increase with the age of the margin these results which are consistent with flexure studies from the major ocean basins have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age the model suggests that a low rigidity plate or airy type model is most applicable early in rifting history while a high rigidity plate is more applicable later in margin evolution thus sediments at a margin initially load a relatively thin and hot plate while sediments later in margin evolution load a relatively thick and cold plate the model explains a number of features of the tectonic stratigraphic evolution of margins these include a change in flexural style from a narrow and deep basin to a broad and shallow basin coastal plain onlap and uplift in the continental interior furthermore the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins in particular the model explains the existance of airy isostatic gravity anomalies at margins the amplitude and wavelength of these anomalies are a function of the sediment supply the type of mechanisms controlling sediment deposition up building and or out building and the thermal structure history and age of the margin

antarctic marine temperatures late campanian through early paleocene an oxygen isotope paleotemperature study of benthic foraminifera from the upper campanian through lower paleocene section at seymour island near the coast of antarctica was done in conjunction with careful scanning electron microscope observations to monitor the state of foraminiferal preservation the isotopic compositions of those samples which were deemed to be well preserved were the basis of the conclusion that campanian through paleocene high latitude shelf waters near antarctica had temperatures between approximately 4 5 and 10 5 c temperatures during early and middle maastrichtian time were slightly higher than those of either the late campanian or late maastrichtian the inference of temperate climatic conditions in antarctica is in agreement with paleoclimatic conclusions drawn on the species composition of the planktonic foraminiferal assemblages and the character of the planktonic foraminifera as well as with published conclusions based upon paleobotanical evidence the maastrichtian and campanian water temperatures at seymour island are similar to those inferred from published isotope ratios of pacific late cretaceous benthic foraminifera from low latitudes indicating that campanian through early paleocene deep ocean waters at least in the pacific and perhaps elsewhere as well could have formed by sinking of surface waters in polar regions

seasonal changes in species composition numbers mass size and isotopic composition of planktonic foraminifera settling into the deep sargasso sea abstract planktonic foraminifera recovered from six successive sediment trap samples collected over fourteen months at a depth of 3200 m near bermuda show varied responses to seasonal changes in the near surface hydrography these include changes in species composition and within species changes in number of individuals collected per unit time average size and weight of individuals and in oxygen and carbon isotopic composition a total of seventeen species were identified and eleven were sufficiently abundant for isotopic analysis in at least some of the samples the accumulations on the sea floor of some of these species can be expected to contain a strong seasonal component in their oxygen isotopic composition which may be utilized in paleoclimatic and paleo oceanographic reconstructions best suited to this purpose are those species which form their tests in isotopic equilibrium with surface water and whose abundance is confined to a portion of the year only the best examples are pulleniatina o bliguiloculata globigerinoides conglobatus and neogloboquadrina dutertrei globigerinoides ruber is a good estimator of the average surface water temperature because it occurs in rather even abundance throughout the year hastigerina pelagica though very abundant in surface water is practically eliminated even before arrival on the sea floor by partial resorption and structural weakening of the tests during gametogenesis orbulina universa globorotalia truncaltulinoides and globorotalia hirsuta show morphological and istopic evidence for contibnued calcification in deeper water after initial test formation near ther surface globigerina bulloides represents the most striking example of oxygen isotopic disequilibrium with sea water

cenozoic marine sedimentation and ice volume variation on the east antarctic craton recycled cretaceous and cenozoic marine microfossils have been recovered from samples of the pliocene sinus formation samples were collected from outcrops in the reedy beardmore and ferrar glacier areas of the transantarctic mountains between lat 77 and 86 s the glaciogene sediments contained diatoms foraminifera calcareous nannoplankton silicoflagellates radiolarians sponge spicules palynomorphs and ostracodes of late cretaceous paleocene eocene late oligocene late miocene and pliocene age this suggests the presence of open marine basins on the east antarctic craton during late mesozoic and cenozoic time the apparent absence of early oligocene and early through middle and earliest late miocene assemblages suggests either that marine regression exposed the basin floors or that ice filled the basins during these times the high elevation setting of sirius formation outcrops suggests one of two hypotheses for their origin 1 they are in situ pliocene glaciomarine deposits that were uplifted 1 750 2 500 m with the transantarctic mountains to their present elevation 2 the sirius formation deposits are a mixture of derived sediments stripped from sub ice intracratonic basins and subsequently redeposited by ice flowing up the inland slope of the transantarctic mountains we favor the second hypothesis with transport to sites sometime within the past 3 m y

late cretaceous paleogene paleogeography and paleocirculation evidence of north polar upwelling abstract quantitative determination of biogenic silica in late cretaceous and paleogene deep sea sediment cores from the central arctic ocean provides evidence of open ocean polar upwelling the timing of polar upwelling coincides with periods of a weakened meridional thermal gradient suggesting that heat transport to the poles by oceanic circulation may have been important the timing of biogenic silica deposition in the arctic precedes its deposition in both the norwegian greenland sea of the north atlantic and he bering sea of the north pacific tectonic events may be resposnible for the timingg and sitting of sites of deposition of biogenic silica in high northern latitudes particularly the tectonic evolution of sites of deep water exchange between the arctic and the world ocean we outline three phases in the post mid cretaceous history of silica deposition in high northern latitudes during phase i the arctic is a silica sink with deep water formation but with no deep water outflow the transition to phase ii is brought about by opening of the svalabard greenland strait to deep water outflow from the arctic to the norwegian greenland sea the transition to phase iii is initiated by submergence of the faroe iceland ridge and deep water outflow from the arctic to the north atlantic climatic conditions in the arctic during late cretaceous and paleogene time are predicted to have favored open ocean upwelling due to a circulation pattern dominated by cyclonic conditions resulting from the establisment of a semi permanent atmospheric low over the alpha ridge bathymetry of the alpha ridge may have intensified paleo upwelling

a paleoceanographic model for the early evolutionary history of planktonic foraminifera the first major radiations of planktonic foraminifera occurred during the mid cretaceous however this period of great diversification was interrupted by two intervals of pronounced paleoceanographic paleoecologic change one during the latest aptian early albian and a second associated with the cenomanian turonian boudary both of these times corresponded with widespread organic carbon burial during intervals commonly referred to as oceanic anoxic events oae 1 and 2 respectively this synthesis focuses on the evolutionary history of aptian early cenomanian planktonic foraminifera and the relationship with oae 1 dynamic paleoceanographic changes controlled the course of planktonic foraminiferal evolution during the mid cretaceous heightened plate tectonic activity a major long term rise of global sea level and an overall increase of global temperatures characterized aptian albian time as proposed here the evolutionary history of aptian early cenomanian planktonic foraminifera was influenced most strongly by changes in the density structure and stability of the upper water column via changes in the sites and rates of water mass production through time and perhaps by subtle changes in oceanic productivity including trophic structure and shifts in productivity centers with rising sea level the creation and growth of epicontinental seas played a major role in regional climate change water mass production and water column structure epicontinental seas were also an important factor in creating vast new niche space and increased biotic diversity for many marine organisms the amplified seasonality and instability of epicontinental seas resulted in habitation of the upper water column by morphologically simple eurytopic planktonic foraminifera several new species of epicontinental sea dwelling planktonic foraminifera appeared during the mid cretaceous however much of the diversification of morphologic form took place in oceanic areas during times of oceanic stability and upper water column stratification during mid to late aptian and late albian time a decline in diversity and return to simple morphotypes occurred when oceanic stability was disrupted and the upper water column was poorly stratified during latest aptian early albian time

a possible explanation of gravity anomalies over mid ocean ridges gravity fields over the crests of slow spreading mid ocean ridges are characterized by small amplitude free air gravity anomaly lows 30 70 mgal with wavelengths of 40 60 km flanked by smaller gravity highs in general the amplitude and wavelength of these gravity lows decrease with increasing spreading rate until at fast spreading ridges 5 cm yr free air gravity highs 10 20 mgal are observed previous explanations of these anomalies which involve uncompensated ridges thermally expanded ridges and elastic plate compensation models are generally not consistent with the geological evidence regarding the nature of the crust and upper mantle beneath the ridge crest in particular the postulated presence of wide zones of partial melt in the uppermost mantle is inconsistent with petrologic data from mid ocean ridge basalts which indicates that primary melts are in equilibrium with and extracted from residual mantle at depths of 30 km or more in addition similar gravity lows observed over extinct spreading ridges suggest that the anomalies are not mainly due to any dynamic aspect of the accretion process taking place at active spreading centers geometrical models of mid ocean ridges derived from detailed structural and petrologic studies of the bay of islands ophiolite complex have been used to generate gravity anomalies over ridges with various spreading rates modeling results show that four main sources serve to define the amplitude and wavelength of the gravity anomaly the ridge topography the crustal level magma chamber the configuration of the isotherms near the ridge crest and a low density partially gabbroic cumulate root extending vertically into the mantle beneath the ridge systematic variations in the contributions from these sources with increasing spreading rate control the changes in character of the gravity anomaly from slow to fast spreading centers major contributions are found to come from the topography and from the lowdensity root which broadens with slower spreading rates resulting in both increasing amplitude and increasing wavelength with decreasing spreading rate at extinct spreading centers contributions from the magma chamber isotherms and topography gradually diminish with time once the ridge ceases to spread whereas the low density root remains a permanent part of the oceanic lithosphere and continues its contribution to the gravity field seismic evidence from slow spreading centers appears to support the existence of the lowdensity gabbroic root zone inferred from the study of the bay of islands ophiolite and the gravity analysis presented here

the structure and origin of prydz bay and macrobertson shelf east antarctica abstract a marine geophysical survey in early 1982 conducted by the australian bureau of mineral resources indicated that much of prydz bay is underlain by a sedimentary basin severe seismic multiples preclude an accurate estimate of total sediment thickness but interpretation of the seismic and magnetic data suggest that it is probably at least 5 km the trends of the southeast basin margin and of mild faulting and folding in the southwest indicate an overall nne trend roughly orthogonal to the continental margin in the south of prydz bay two series of seismic sequences are evident separated by a mildly erosional unconformity the lower series ranges from poorly to well stratified has minor folding and faulting and probably derives from continental and perhaps shallow marine pre breakup sediments the upper series is generally well stratified and prograded near the shelf edge it probably represents shallow marine post breakup sediments the seabed is distinctly unconformable with the underlying sediments implying both that much of the upper series sediments and some of the lower series sediments have been bulldozed off by advances of the amery ice shelf and that present sedimentation rates are very low possible thin moraines or tillites in the northeast part of the prydz bay are also attributed to these glacial advances the continental slope and rise sedimentary section ranges from at least 3 km thick off prydz bay to thin off the macrobertson shelf to the west reflecting the more prolific sediment source in prydz bay the deep water section includes several seismic sequences the most distinctive being interpreted as sheet volcanics and turbidite fans the deepest visible unconformity is locally strongly faulted and may separate the pre breakup and post breakup sediments indo antarctic breakup has been tentatively dated as early neocomian 130 ma and the e w orientation of the resultant antarctic coast invites interpretation of the lambert glacier prydz bay structure as a possible failed rift arm of a triple junction direct information on the age and nature of the sediments under prydz bay is lacking but permian continental sediments cropping out at beaver lake to the south may correspond to the lower pre breakup series this interpretation gains some support from analogies with fault bounded intracratonic basins in india which contain permian to triassic continental strata and which may have been juxtaposed prior to breakup the upper series probably consists of upper cretaceous and cainozoic sands and shales with moraines or tillites at the top of the section

isostatic response of the central indian ridge western indian ocean based on transfer function analysis of gravity and bathymetry data abstract the central indian ridge cir is a part of the major active mid ocean ridge system trending north south between 2 n and the triple junction at 25 s previous investigations suggest that the half spreading rate varies from 1 6 to 2 5 cm yr between the northern and southern ends of the ridge and the diffuse plate boundary dividing the indian and australian plates intersects the central indian ridge near the equator in the present study i have attempted to investigate the nature of isostatic response of young oceanic topography created by spreading along the central indian ridge gravity and bathymetric profiles four in northern part of the ridge within or close to the diffuse boundary zone and six in southern part outside the zone have been utilized separately to obtain their isostatic response based on transfer function analysis the admittance estimates for the long wavelengths suggest that the elastic plate model fits the data for the compensation of sea floor topography near the ridge crest an effective elastic thickness ranging from 5 to 10 km with an average t e of 7 2 km fits best to the data for the northern part of the central indian ridge and a value from 10 to 17 km with an average t e of 13 3 km fits the data for the southern part of the ridge isostatic response function estimates confirm that the difference in elastic plate thicknesses between the northern and southern segments is real and significant the observed low effective elastic thickness for the data in the northern part of the central indian ridge close to the diffuse boundary zone is interpreted as due to higher than average temperatures beneath the ridge these higher temperatures have been inferred due to the perturbations of the regional heat flow anomaly associated with the diffuse plate boundary

lithospheric structure of a nascent spreading ridge inferred from gravity data the western gulf of aden the aden spreading ridge somalia arabia plate boundary does not connect directly to the red sea spreading ridge it propagates toward the east african rift through the afar depression where the presence of a hot spot has been postulated from seismological and geochemical evidence the spreading direction n37 e is highly oblique to the overall trend n90 e of the ridge we present and interpret new geophysical data gathered during the tadjouraden cruise r v l atalante 1995 in the gulf of aden west of 46 e these data allow us to study the propagation of the ridge toward the afar and to discuss the processes of the seafloor spreading initiation we determine the lithospheric structure of the ridge using gravity data gathered during the cruise with the constraint of available refraction data a striking bouguer anomaly gradient together with the identification of magnetic anomalies defines the geographical extent of oceanic crust the inversion of the bouguer anomaly is performed in terms of variations of crustal thickness only and then discussed with respect to the expected thermal structure of the mantle lithosphere which should depend not only on the seafloor spreading but also on the hot spot beneath east africa our results allow us to define three distinct lithospheric domains in the western gulf of aden east of 44 45 e the lithosphere displays an oceanic character thermal subsidence recorded for the last 10 ma and constant crustal thickness between 43 30 e and 44 10 e the lithosphere is of continental type but locally thinned beneath the axial valley the central domain defined between 44 10 e and 44 45 e is characterized by a transitional lithosphere which can be seen as a stretched continental crust where thick blocks are mixed with thinned crust it displays en echelon basins that are better interpreted as extension cells rather than accretion cells

crustal structure and isostatic compensation near the kane fracture zone from topography and gravity measurements i spectral analysis approach summary six gravity and bathymetry profiles perpendicular to the kane fracture zone each more than 300 km long were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages a spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths for remaining wavelengths the admittance the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate about 6 km in thickness after subtracting the gravitational attraction of the bathymetry and its compensation substantial isostatic anomalies remain we interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography except at very long wavelengths the apparent crustal thickness varies by as much as a factor of 2 but there is no evidence indicating systematic thinning of the crust beneath the fracture zone our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface this indicates that there are two different modes of crustal formation with different gravity and topographic signatures effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies and intrusive volcanism or underplating producing gravity anomalies but little topographic relief

late cretaceous eocene nannofossil and magnetostratigraphic correlations near gubbio italy abstract using a modified sample preparation technique we have been able to establish a detailed lower campanian to upper eocene nannofossil stratigraphy in the bottaccione and contessa highway sections near gubbio appearance and extinction levels of virtually all the commonly used calcareous nannofossil zonal markers have been recognized and can now be closely correlated with the planktonic foraminifera zonation and the magnetic reversal stratigraphy previously established in these sections comparisons with the nannofossil calibrations of the oceanic magnetic anomaly sequence in deep sea drilling project dsdp sites suggest that magmetic subchrons c17n and c25n are missing in the bottaccione section the observed variability of the relative stratigraphic position of most plankton events is confirmed to less than one magnetic subchron absolute abundance paleobiogeographic restriction and differential preservation render some of the traditionally used biostratigraphic events less reliable than others

biogeographic patterns in terminal cretaceous planktonic foraminifera from tethyan and warm transitional waters abstract biogeographic patterns in terminal cretaceous planktonic foraminifera have been analyzed from their faunal compositions at a time slice spanning the upper parts of the calcareous nannofossil micula prinsii zone approximately the latest 20 60 k y of the cretaceous at dsdp sites from low 16 through middle 37 paleolatitudes in either the northern or the southern hemisphere the study is based on relative abundance data of 26 species at sites 356 516f 525a and 527 from the south atlantic ocean sites 384 and 548a from the north atlantic and sites 465a and 577a from the north pacific cretaceous tertiary k t boundary sections at site 516f and 577a are poorly preserved which prevented quantitative faunal analysis two major faunal assemblages one tethyan assemblage and one transitional assemblage are distinguished from the quantitative distribution patterns the tethyan assemblage dominating at sites 465a 356 and 384 is composed of heterohelix striata globotruncana aegyptiaca pseudoguembelina kempensis p costulata an undescribed form of heterohelix with relict apertural flanges along medium suture rugoglobigerina hexacamerata and trinitella scotti among these species p kempensis p costulata and h sp exhibit enhanced abundances at site 465a and constitute a warm tethyan subassemblage whereas r hexacamerata and t scotti which are most abundant at sites 356 and 384 represent a cool tethyan subassemblage seven species show greater relative abundances at sites 525a and 527 and are referred to a warm transitional subassemblage abathomphalus mayaroensis gublerina cuvillieri pseudotextularia elegans rugoglobigerina rugosa racemiguembelina fructicosa globotruncana arca and planoglobulina acervulinoides in addition an assemblage marked by the heterohelix glabrans h carinata complex h globulosa globigerinelloides multispina and rosita contusa may represent a shallower water subassemblage within warm transitional waters site 548a

latitudinal variation in the planktic foraminifer contusotruncana contusa in the terminal cretaceous ocean abstract the morphological variability coiling properties size and shape of the planktic foraminifer contusotruncana contusa cushman in the terminal cretaceous ocean was examined at eight deep sea sites and two continental sections from low 16 to middle 42 paleolatitudes in both hemispheres the material used in this study includes samples from the south atlantic dsdp sites 356 527 and 525a north atlantic sites 384 and 548a indian and pacific oceans dsdp site 465a and odp sites 761c and 762c and tethyan ocean outcrop sections from el kef and caravaca on average 45 specimens from two samples per location were analysed from an interval corresponding approximately to the last 60 kyr of the cretaceous no differences in coiling direction dextral proportions were 90 in all samples percentage of kummerform specimens usually 50 and number of chambers in the last whorl 4 5 were observed between the sites both test size expressed as spiral outline area and test volume and total number of chambers increase significantly towards lower latitudes similarly test conicity examined by shape coordinate and eigenshape methods and angularity of the spiral outline show a rather continuous slight increase towards lower latitudes kummerform specimens of c contusa were slightly larger and more conical than normalforms and possessed substantially more chambers both totally and in the last whorl a principal components analysis of the sample means of five variables describing size and shape clearly distinguished high latitude sites 525a 527 548a 761c and 762c from low latitude sites 384 465a caravaca and el kef specimens from site 356 are transitional with respect to those two groups the results indicate 1 considerable morphological variation in c contusa during the terminal cretaceous comparable to that known in many recent planktic foraminiferal species and 2 a geographical distribution of this variation corresponding to previously suggested biogeographic schemes based on quantitative analysis of planktic foraminiferal assemblages despite the differences in sample means the overall morphology of c contusa overlaps among the sites studied supporting the classification of all c contusa morphotypes as a single species similarly no discrete morphologic groups could be distinguished within any of the samples

late mesozoic sedimentation in the northern antarctic peninsula and its relationship to the southern andes two distinct phases of island are evolution are recognized in late mesozoic sedimentary rocks of the northern antarctic peninsula during late jurassic kimmeridgian tithonian times alternating radiolaria rich mudstones and ash fall tuffs were deposited under shallow marine euxinic conditions in association with restricted volcanic centres the name nordenskjold formation is proposed for this lithostratigraphic unit subsequently during the early cretaceous a major episode of volcanism and uplift led to the construction of an emergent are terrane simultaneous development of a retro arc basin resulted in the accumulation of coarse volcaniclastic detritus along the eastern side of the arc retro are sedimentation and intermittent volcanism continued into the late cretaceous and early tertiary although the pattern of sedimentation in the northern antarctic peninsula is broadly comparable to that of the southern andes an active marginal basin has not been recognized in the former area interbedded mudstones and tuffs identical to those of the nordenskjold formation were deposited in the southern andes and south georgia during the late jurassic early cretaceous

faunal turnover at the end of the cretaceous in the north pacific region implications from combined magnetostratigraphy and biostratigraphy of the maastrichtian senpohshi formation in the eastern hokkaido island northern japan abstract a combined magnetostratigraphic and biostratigraphic study has been performed on the maastrichtian senpohshi formation in eastern hokkaido island northern japan which is an approximately 1300 m thick section mainly composed of hemipelagic mudstone the identification of magnetic polarity was possible at 51 horizons whereby four magnetozones were recognized these magnetozones were correlatable to geomagnetic polarity chrons c31r to c30n suggesting that the age of the senpohshi formation is spanning from middle to upper part of the maastrichtian ca 69 67 ma the magnetostratigraphy of the senpohshi formation established in this study enables a direct age correlation to the maastrichtian successions in other regions thus this detailed chronology of the formation contributes to paleontological studies of the maastrichtian in the north pacific region for instance this magnetostratigraphic age assessment implies the following 1 the stratigraphic range of the ammonite pachydiscus flexuosus contains polarity chrons from the lower part of c31r to the lower part of c31n 2 the first occurrence fo of the calcareous nannofossil nephrolithus frequens in the north pacific region is correlatable to polarity chron c30n or below and 3 the fo of the bivalve inoceramus awajiensis is located within polarity chrons from c31r to the upper part of c31n this suggests that the inoceramid extinction event in the north pacific region might have occurred during polarity chrons from c31r to the upper part of c31n ca 70 5 67 8 ma which is 2 3 5 0 myr prior to the cretaceous paleogene boundary the trend of the maastrichtian faunal turnover in the north pacific is well consistent with those of other regions brings a new evidence for understanding the global faunal turnover in the maastrichtian just before cretaceous paleogene mass extinction

distribution and provenance of recycled palynomorphs in surficial sediments of the ross sea antarctica abstract quantitative analysis of surficial sediments in the ross sea has shown that the highest densities of recycled palynomorphs occur in a region in the southeast immediately north of the ross ice shelf in the vicinity of roosevelt island there spores and pollen of late cretaceous to early tertiary age predominate eocene dinoflagellates and rare palaeozoic spores are also present in the western half of the ross sea densities of recycled palynomorphs are lower but relative frequencies of palaeozoic types are higher on the continental shelf the palynomorphs have been recovered in equal abundance from sediments identified as basal tills and compound glaci marine sediments comparison of the observed patterns of distribution with patterns of ice flow in the ross ice shelf shows a clear correlation between the high palynomorph densities in the southeastern ross sea and discharge from ice streams d and e geophysical information from aeromagnetic and radio echo sounding surveys in the catchment areas of these ice streams suggests the most likely source for the recycled palynomorphs to be in a region of subdued topography inland of the present grounding line in the marie byrd land sector of the ross ice shelf extending probably not further east than 130 w this region probably represents a continuation of the easternmost ross sea basin the presence of marine eocene sediments there attested to by dinoflagellates in the recycled assemblages accords with suggestions of a late mesozoic cenozoic opening of the ross sea the observed patterns of palynomorph distribution reflect input from ice drainage areas in both east and west antarctica the preservation of sediments of differing provenance in distinct areas on the sea floor probably indicates processes of meltout of debris near the grounding line of the ross ice shelf during past phases of expansion marine bottom currents have not significantly redistributed the recycled microfossils

paleobiogeography of campanian maastrichtian foraminifera in the southern high latitudes abstract distributions of planktonic and benthic foraminifera have revealed antarctic land sea distributions oceanic surface gyre configurations and changes in the configurations of the tethyan transitional and austral realms during campanian maastrichtian time poleward changes in the diversity of total and keeled planktonic species of the southern hemisphere are compared for the early and late campanian and early and late maastrichtian latitudinal diversity gradients were weakest during the early campanian and more pronounced during the late campanian and maastrichtian shallow marine seaways within west antarctica are suggested by similarity of late cretaceous nearshore benthic and open ocean planktonic assemblages from southern high latitude land and drill sites and by the occurrence of recycle cretaceous marine microfossils from antarctic interior and continental margin localities development of a major water mass boundary between cool surface waters south of about 50 s paleolatitude and warmer surface waters to the north in the south atlantic and indian ocean may have caused biogeographic isolation of austral realm assemblages during late campanian and early maastrichtian time several planktonic foraminifera show significant diachroneity in their latitudinal distributions magnetobiostratigraphic data suggest that abathomphalus mayaroensis migrated from high to low latitudes whereas pseudotextularia elegans globigerinelloides subcarinatus globotruncanella citae globotruncanella petaloidea globotruncana bulloides and globotruncana subcircumnodifer made time transgressive poleward migrations the distribution of a mayaroensis is in agreement with isotopic evidence for late maastrichtian climatic cooling while that of p elegans correlates with a latest maastrichtian warming event about 66 8 66 6 ma delayed first occurrences of the other taxa cannot be explained by paleoclimatic factors alone variations in global eustatic sea level and orogeny between southern south america and west antarctica may have also influenced the taxonomic composition of southern high latitude foraminiferal assemblages

stable isotope evidence for foraminiferal habitats during the during of the cenomanian turonian ocean anoxic event oxygen and carbon isotope analyses of several species of size controlled planktonic and benthic foraminifera from the plenus marls of dover kent england show a clear separation in oxygen and carbon isotope ratios consistent with different habitats the analyses indicate the existence of vertical gradients of temperature and 13 c in dissolved co 2 in the chalk sea whereas sequential ontogenetic stages of planktonic foraminifera do not show systematic variations in 18 o or 13 c values implying no systematic change in depth habitat during the growth of individual species species of the genera hedbergella and dicarinella are interpreted to have lived nearer to the surface than rotalipora because vertical 13 c gradients in the chalk sea remained stable during the late cenomanian the oxygen minimum zone is unlikely to have impinged on this area

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

biochemostratigraphy of an upper albian turonian succession from the southeastern neo tethys margin sw iran abstract a neo tethyan upper albian turonian hemi pelagic carbonate succession sarvak formation accessible in outcrop in the zagros sw iran was investigated using sedimentology microfacies analysis planktonic foraminifera biostratigraphy and carbon isotope stratigraphy the succession contains six biozones including thalmanninella th appenninica th globotruncanoides rotalipora cushmani whiteinella archaeocretacea helvetoglobotruncana helvetica and marginotruncana schneegansi some of these biozones show variable stratigraphic positions throughout the tethyan and boreal realms that hinder correlation of the studied iranian section with european reference sections therefore the correlation of the existing biostratigraphic framework was complemented by carbonate carbon isotope stratigraphy based on bulk carbonate material despite the low resolution of the presented carbon isotope record pronounced global events including the late turonian events cenomanian turonian boundary ctb event oae 2 and albian cenomanian boundary acb event can be distinguished the 13c excursions provide global synchronous time lines for correlation the distinguished biozones were compared among the sections regarding their stratigraphic positions relative to these time lines the correlation tentatively suggests that the biostratigraphic datum marking the acb is diachronous with an se nw trend first appearance datum fad of th appenninica varies from the upper albian to the lower cenomanian the fad of th globotruncanoides is also variable from the upper albian to the lower cenomanian however the middle cenomanian turonian biozones including rotalipora cushmani and whiteinella archaeocretacea zones are synchronous elevated pco2 contributed to the gradual increase of the sst sea surface temperature during the cenomanian and the dramatic climax at the earliest turonian this trend closely correlates with the change from the diachronous acb biozones to synchronous ctb biozones possibly increasing ssts provided temperature tolerance range for the species throughout different latitudes and assisted their rapid distribution moreover the warming raised the sea level from the latest albian to turonian that inundated landbridges resulting in an enhanced marine circulation and faster distribution of planktonic species throughout the neo tethys ocean

the gulf of aden structure and evolution of a young ocean basin and continental margin new marine geophysical data are used to describe the structure and history of the gulf of aden magnetic anomaly data shows seafloor spreading magnetic anomalies of sheba ridge from the axial anomaly to anomaly 5 10 m y b p between the owen fracture zone and 45 sup 0 e and to anomaly 2 3 m y b p or anomaly 3 4 m y b p west of 45 sup 0 e the data does not support the two episodes of seafloor spreading recently proposed landward of the seafloor spreading magnetic anomalies is a magnetic quiet zone of uncorrelatable anomalies the magnetic quiet zone boundary is also a structural boundary effectively marking the edge of sheba ridge with deeper basement lacking a significant topographic gradient found on the landward side a magnetic quiet zone is found not only where sheba ridge splits continental lithosphere but also on east sheba ridge where the ridge splits the old oceanic lithosphre of the owen and somali basins there the position occupied by the continental margin within the gulf is marked by nonmagnetic ridge complexes that stretch from the continents to the owen fracture zone the magnetic quiet zone boundary is not an isochron in either the gulf of aden ormore the red sea suggesting that significant horizontal motions can occur prior to the initiation of seafloor spreading the offset on the dead sea rift is used to estimate that from 80 to 160 km of opening amounting to between 65 and 200 extension of the initial rift valley occurred in the gulf of aden and red sea prior to the establishment of a mid ocean ridge it is suggested that the development of a new ocean basin occurs in two stages the first involves diffuse extension over an area perhaps 10 km wide in a rift valley environment without an organized spreading center this is followed by concentration of the extension at a single axis and the beginning of true seafloor spreading less

interspecies variation in stable isotopic signals of maastrichtian planktonic foraminifera we measured stable isotopic signals of the largest specimens of all abundant species of planktonic foraminifera in late maastrichtian samples from three atlantic mid latitude and high latitude sites each sample is marked by a relatively small interspecies range of 18o values 1 2 to 0 8 each also exhibits an interspecies range of 13c values that resembles those of modern assemblages at similar latitudes 2 8 to 0 9 with greater ranges at lower latitudes these results indicate that late cretaceous planktonic foraminifera inhabited water masses characterized by a relatively narrow range of temperatures but may have occupied an array of niches as ecologically diverse as that of modern taxa maastrichtian taxa exhibit more complex relationships between skeletal morphology and vertical and seasonal paleohabitat affinities than previous paleoecologic models have assumed for example keeled taxa were not limited to cold or deep habitats and serial taxa appear to have occupied both warm near surface niches and the coolest or most saline habitats the isotopic differences between species indicate that apparent variation in maastrichtian stable isotopic signals strongly depends on the taxa analyzed narrowly constrained size fractions of rugoglobigerina species may provide a good estimate of relative variation in isotopic equilibria of near surface summer paleoconditions at low and middle latitudes gublerina species laeviheterohelix species and planoglobulina multicamerata appear the likeliest candidates for consistently tracking relative variation in isotopic equilibria of the coldest or most saline paleohabitats of maastrichtian planktonic foraminifera these isotopic results suggest that upper water column thermal gradients may have been only 3 to 5 c in mid latitude and high latitude regions they also suggest that the western and central atlantic mid latitude summer seasurface may have been much cooler or saltier in the maastrichtian than it is today finally they corroborate previous estimates of 10 c surface water at southern high latitudes

an application of the response function technique to profiles of bathymetry and gravity in the gulf of aden summary the relationship between bathymetry and gravity in the eastern and the western gulf of aden is analysed by computing response functions the form of the response functions indicates that the short wavelength topography is isostatically compensated the most appropriate mechanism of compensation is one in which the topography is not locally compensated but regionally compensated by flexure of the lithosphere though the sets of profiles from the eastern and the western gulf of aden are separated by only 500 750 km the observed response functions are very different the lithosphere in the eastern gulf of aden can be modelled by an elastic plate 15 3 km thick whereas an elastic plate 3 2km thick is probably an appropriate model for the lithosphere in the western gulf of aden a different mechanism of isostasy the pratt operates in the compensation of the long wavelength topography of the mid ocean ridge in the gulf of aden a larger negative broad bouguer gravity anomaly across the western gulf of aden than across the east indicates that upper mantle temperatures increase from east to west the long wavelength shallowing of the gulf of aden and increasing elevation of the adjacent continents from east to west are therefore probably mainly the result of thermal uplift the subdued nature of the median valley and consequently other short wave length topography in the west can be explained by hot low viscosity asthenosphere and the inferred thin lithosphere in the west also suggests high temperature mantle finally a magnetic anomaly superimposed on the axial anomaly which is probably related to the depth of the curie temperature is more common in the western gulf of aden than in the east and this is also consistent with upper mantle temperatures increasing to the west there are a number of similarities between the mid ocean ridge in the western gulf of aden and the reykjanes ridge it is suggested that the western gulf of aden is anomalous because of its proximity to the afar hot spot in the same way that the reykjanes ridge is often considered to be anomalous because of its proximity to the iceland hot spot

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

mass wasting events and their impact on the formation and preservation of submarine ore deposits abstract mass wasting and landform modifying events have a profound impact on hydrothermal processes in terrestrial environments mass wasting events in submarine settings also modify hydrothermal systems and their associated mineralisation we present evidence of a dynamic environment impacting on ore formation at the historically exploited pb zn ag mineralisation of triades milos island greece formed in a submarine setting galena sphalerite veins and barite quartz gangue precipitated in the near subsurface or after exhalation of boiling hydrothermal fluids field evidence indicates that mineralisation was extensively reworked by debris flow events during formation the mineral paragenetic sequence is consistent with a pb zn ag massive sulphide system and analogous to the early stages of a kuroko type deposit but triades lacks massive sulphide bodies we suggest that mass wasting events literally truncated the developing mineral deposit as it formed on the seafloor destroying massive sulphide bodies and limiting the development of the ore mineral assemblages mass wasting processes in volcanogenic massive sulphide systems are ore destructive with little opportunity for telescoping unlike terrestrial equivalents shallow marine systems in terrains subject to mass wasting may have low preservation potential or may be classified as epithermal like vein systems rather than stockwork portions of massive sulphide deposits

the magnetic quiet zone in the eastern gulf of aden implications for the early development of the continental margin summary the magnetic quiet zone in the eastern gulf of aden is located between the oceanic crust of sheba ridge and the continental crust of arabia and somalia and is separated from both by important structural boundaries the seaward boundary is marked by the end of the seafloor spreading magnetic anomaly sequence and by a basement depth discontinuity the landward boundary is marked by escarpments made up of a series of normal faults these escarpments extend from 2 3 km below sea level to 1500 m above sea level and are equivalent of the hinge zone found at mature continental margins the magnetic field in the quiet zone is flat in some areas and in others is characterized by anomalies of up to several hundred gammas which are correlatable over distances of up to about 20 km and which appear related to basement topography the basement lacks the topographic slope characteristic of mid ocean ridge flanks and is characterized by moderately rough relief the crustal structure appears quite heterogeneous and where the crustal thicknesses have been determined they are slightly greater than those of oceanic crust new heat flow measurements show high values 95 7 123 3 mw m 2 in the quiet zone with values decreasing from sheba ridge toward the coast the unusual structure of the quiet zone and the observations that more opening appears to have occurred between arabia and somalia than can be accounted for by the oceanic crust of sheba ridge leads to the suggestion that the magnetic quiet zone was generated by diffuse extension of continental crust through a combination of rotational listric faulting and dyke injection this possibility is investigated using both a stretching or lithospheric attenuation model and a model in which a portion of the extension occurs through dyke injection it is found that these models can adequately match the observed heat flow and basement depths although very large amounts of extension 4 6 are required in the deep seaward portion of the quiet zone this results in more extension than is compatible with the documented motion between arabia and africa however formation of the magnetic quiet zone occurred over a period of 10 15 myr rather than instantaneously as assumed in the simple models when the effects of a finite length rifting episode are considered less extension is required and the observed geophysical data are consistent with a diffuse extension origin for the magnetic quiet zone

mesozoic cenozoic history of calcareous nannofossils in the region of the southern ocean abstract the mesozoic cenozoic evolution of high southern latitude calcareous nannofossil assemblages developed in parallel with that of the southern hemisphere ocean basins and the attendant progressive changes in global climate the equitable middle to late jurassic climates are reflected in a cosmopolitan zonation although distinctive high latitude taxa developed in both polar regions monospecific or low diversity provincial assemblages characterized the incipient early cretaceous south atlantic basins until the introduction of tethyan elements during the aptian bipolar or austral provincial taxa are useful in erecting a biostratigraphic subdivision for the remainder of the cretaceous sequence on the falkland plateau in contrast to the middle cretaceous the global sample coverage of dsdp holes for the campanian maestrichtian is sufficiently broad to demonstrate a distinct latitudinal distribution of species and assemblages provincial austral or bipolar taxa are relatively minor components of the paleocene eocene assemblages but quantitative methods document the existence of definable high latitude assemblages which shifted latitudinally in response to climatic changes indirect evidence suggests that calcareous nannofossils were deposited in the interior basins of antarctica during this interval species diversities declined progressively through the oligocene miocene epochs in response to the deterioration of neogene climates and glacial episodes on the antarctic continent finally resulting in low diversity assemblages generally dominated by high latitude taxa sharp migrations of these assemblages occurred in response to glacial climatic events plio pleistocene sediments deposited beneath the expanding antarctic water mass are essentially barren of calcareous nannofossils but coccoliths are widely distributed in the subantarctic region as a result of the amelioration of holocene climate and the quaternary descent of the carbonate compensation depth two new taxa are described stephanolithion bigotii brevispinus n ssp from the upper jurassic and gephyrobiscutum diabolum n g n sp from the lower campanian

cretaceous and tertiary fossils in glacio marine strata at cape melville antarctica glacio marine sediments of the cape melville formation king george island south shetland islands west antarctica contain a rich and predominantly invertebrate fauna the occurrence of nannoplankton and belemnites might be used as evidence for dating the glacio marine cape melville formation as cretaceous however the co occurrence of the coral genus flabellum eocene recent and the foraminiferal genus uvigerina eocene recent indicate that recycling of the nannoplankton and belemnites has occurred the sediments of the cape melville formation contain a variety of glacially striated ice rafted dropstones up to 2 m of antarctic continent provenance these sediments lie between lavas of the sherratt bay formation and pliocene polonez cove formation and may correlate with a late miocene glaciation of antarctica

global cretaceous paleogeography international geologic correlation program project 191 abstract five global cretaceous paleogeographic maps within a plate tectonic framework are illustrated based on the ten contributions in this special volume

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

on the quality of marine magnetic anomaly sources and sea floor topography summary the quality of marine magnetic anomaly sources is described with the power density representation of a stochastic model of random temporal and spatial emplacement of the marine magnetic anomaly source in the oceanic crust typical values of sea floor spreading and emplacement parameters define a high fidelity process of recording and sea surface detection of the palaeomagnetic field reversals for spreading rates over 20mmyr an analogous stochastic model is developed for the formation of sea floor topography by normal faulting it is shown that the random process of normal faulting observed in the inner walls of the famous rift valley can account for the quality of the adjacent west rift mountains topography quality as opposed to quantity is a degree of excellence relative nature or kind or character concise oxford dictionary of clcrrent english the signal to noise ratio is a measure of quality in this paper we address the signal to noise ratio of sea floor spreading magnetic anomaly sources we use a stochastic model of discrete random temporal and spatial emplacement of magnetic source material in a spreading centre that records the ambient geomagnetic field behaviour and show that the temporal aspect i e the mean frequency of emplacement in a spreading centre is critical to the quality of sea floor spreading magnetic anomaly sources it will be shown that the sea floor topography generated by a stochastic process of extension or thinning of the brittle crust in response to discrete and random normal faulting in atspreading centre can be treated as noise the quality of sea floor topography can be estimated and expressed in terms of mean fault density and crustal thinning as a measure of the tectonic processes which formed the topography

mapping the crust mantle boundary beneath afar depression abstract the funnel shaped afar depression in the horn of africa situated at the separation zones of the nubian somalian and arabian plates represents a triple junction between the main ethiopian rift the red sea and gulf of aden rifts this depression occupies an important place in that it is the only emerged triple junction in the world assuming that the mechanism of compensation is one in which the topography is regionally compensated by the flexuring of the elastic plate and that linear relationship exists between the gravity field of the compensation and the topography isostatic correction isostatic anomaly and moho depth maps are computed for the depression the results show that nonlinear elements are small in afar and hence the crust mantle interface beneath afar modelled using admittance technique compares very well with the existing seismic results the results also show that there is crustal attenuation to the north and east and that there is an under compensation in the north that may be due to the upwelling of mantle material along the active axis

gulf of aden axial magnetic anomaly and the curie temperature isotherm the main features of magnetic anomalies over ocean ridges have been explained1 as a corollary of seafloor spreading and geomagnetic reversals oceanic crust is formed in a narrow region becoming magnetized in the direction of the earth s magnetic field as its temperature falls through the curie point of the magnetic minerals present the gulf of aden was one of the first places where reversely magnetized sea floor was recognized2 the seafloor spreading direction and latitude are such that the anomaly due to normal magnetization is negative and slightly skewed positive anomalies were also observed suggesting the presence of reverse magnetization a short wavelength magnetic anomaly which frequently occurs superimposed on the axial magnetic anomaly in the gulf of aden is now described various interpretations are considered the preferred involves a dramatic shallowing of the curie temperature isotherm close to the seafloor spreading centre the interpretation has implications for models of the generation of oceanic lithosphere and for locating possible geothermal areas in rifted regions

late mesozoic bivalve biostratigraphy of the antarctic peninsula region two groups of bivalves the inoceramids and buchiids are particularly useful zone fossils in the mesozoic sedimentary formations of the antarctic peninsula region in the upper jurassic species of the genera retroceramus malayomaorica buchia and anopaea can be traced considerable distances around the margins of gondwana and as such are especially valuable for regional correlations some of them such as the r haasti subhaasti group can also be employed in detailed local correlations in the lower cretaceous the presence of cosmopolitan inoceramus species such as members of the neocomiensis concentricus and anglicus groups permits correlation with northern hemisphere faunas for the first time other stratigraphically important inoceramids are the i ovatus group in the lower neocomian the i heteropterus group and anopaea trapezoidalis in the upper neocomian and an cf mandibula in the albian the only buchiids present in the lower cretaceous of antarctica belong to the extensive aucellina andina radiatostriata group two types of antarctic upper cretaceous inoceramids that are particularly important for regional correlations are the upper cenomanian i pictus group and the turonian coniacian i madagascariensis ernsti group senonian inoceramids are still poorly known but would seem to offer considerable scope for further studies

origin and tectonic evolution of the caroline ridge and the sorol trough western tropical pacific from admittance and a tectonic modeling analysis abstract an independent caroline sea plate surrounded by prominent seismotectonic features is said to exist in the western tropical pacific the caroline sea plate is a transient feature existing only when the caroline philippine sea plate boundary is jumping to the caroline pacific boundary the origin and the tectonic evolution of the caroline ridge sorol trough region which forms the northern part of the caroline pacific plate boundary are examined using a standard admittance analysis a tectonic modeling study and additional direct gravity and magnetic analyses the observed admittance function is fitted well by shallow airy model in the 250 125 km 0 025 k 1 wavelength region whose average crustal thickness t c is 20 km and also by thin plate flexure model of isostasy within the diagnostic waveband of 250 80 km 0 025 k 1 with an effective elastic thickness t e of 5 7 km a simple interpretation of the observed admittance function is that the local airy model of isostasy is the main support mechanism in the study region for the longer wavelength more fundamental topography corresponding to that of the caroline ridge the caroline ridge is inferred to have formed in a near ridge tectonic setting on top of oceanic lithosphere which was young thin and weak at the time of emplacement the tectonic model study after an explanation for rifting along the mendana fracture zone in the nazca plate suggests that the sorol trough formed in the 17 7 ma interval splitting the caroline ridge into two longitudinal halves the rifting along the sorol trough which took place in the manner described in weissel and karner 1989 flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension j geophys res 94 13 919 13 950 necking of unbroken lithosphere post dated the formation of the caroline ridge by 10 15 m y during which time t e in the region increased to 5 10 km the rifting necking along the sorol trough represents a second loading event in the region the combined gravity effects of the rifting as well as the increase in the t e in the region from essentially nil to 5 10 km is reflected in the agreement between the observed admittance and the regional model with a 5 7 km t e in the wavelength region 80 km these results are consistent with the findings and the suggestions of the previous investigators in the region in that 1 the caroline ridge was constructed in the late oligocene by hot spot volcanism on top of oligocene seafloor of the caroline sea back arc basin 2 the eauripik rise genetically related to the caroline ridge formed on young and weak caroline basin lithosphere and is supported by airy manner crustal thickening and that 3 the lithosphere of the caroline ridge was young at the time of rifting necking along the sorol trough on the basis of the current results and the findings of the previous workers a tentative hypothetical rheological model for the crustal density structure of the caroline ridge sorol trough is also given the caroline sea plate appears to be bounded in the north and south by e w trending boundaries that deform diffusely while more normal narrow n s trending boundaries exhibiting more localized albeit slow deformation bound the plate in the east and west

crustal structure of atlantic fracture zones i the charlie gibbs fracture zone summary a seismic refraction experiment was carried out in 1982 over and in the vicinity of the southern transform valley of the charlie gibbs double fracture zone to measure the rate of crustal thinning normal to the southern transform valley axis three reversed lines were shot with obs two were east west one parallel to the fault on normal crust and one along the fault the third line was north south normal to and intersecting the first two lines travel times were modelled by ray tracing thin 4 km thick anomalously low velocity crust was detected in the fracture zone along the n s line normal crust thinned over a distance of 40 km from 8 to 5 km towards the fracture zone where the belt of anomalous crust was 12 km wide several publications demonstrate that such thinning is not due primarily to the effect of cold lithosphere on accretion at the ridge transform intersection the thinning may be related to the supply and distribution of magma from an injection zone situated roughly at the centre of each spreading segment a gravity profile along the n s line was also analysed the bouguer anomaly was modelled subject to the constraints of the seismic ray tracing after applying corrections for the three dimensional bathymetry near the profile and for the lithospheric anomaly due to the age contrast across the northern fracture zone the gravity models supported the seismic interpretation and also indicated low densities within the fracture zones two alternative models were obtained for the structure under hecate bank an east west ridge which lies within the crust generated at the short central spreading segment of the cgfz one model features dense material within the crust the other normal but uplifted crust the evidence favours the latter mode but the cause of the uplift is uncertain the uplift may have

structure and development of the midocean ridge plate boundary in the gulf of aden evidence from gloria side scan sonar a gloria side scan sonar survey along the crest of the midocean ridge in the gulf of aden has provided information on the small scale morphology of its spreading centers and fracture zones spreading center offsets increase in length to the east this is thought to result from changes in the behavior of the mantle with increasing distance from the afar hot spot a 300 km long spreading center in the western gulf of aden is oriented obliquely to the spreading direction nested within the obliquely oriented median valley is an en echelon arrangement of a number of smaller elongated basins each oriented approximately normal to the spreading direction close to the axis of spreading volcanic and closely spaced tectonic structures run approximately normal to the spreading direction higher on the walls of the median valley more widely spaced normal faults have formed subparallel to the valley in the western gulf of aden there are no indications of sonar targets that might represent transform faults parallel to the nne spreading direction in the east spreading center offsets of 15 25 km are made at fracture zones which are best described as narrow zones of highly oblique spreading offsets of greater than 25 km are achieved on single transform faults and the associated tectonic fabric has a spreading direction trend the boundaries between crusts of different ages along inactive transform fault extensions lie along the bases of fracture zone scarps these scarps are situated in the older crust and face the younger this is the opposite of the situation predicted on the basis of simple plate cooling models at some places in the gulf of aden the fracture zone topography is very subdued and this is attributed to the failure of the two slabs of lithosphere at the end of the transform fault to couple completely before the younger lithosphere on the nontransform side of the spreading center completes its ascent of the median valley wall there is evidence of a change of spreading direction 3 5 m y ago from about 040 to 022 it is argued that the small scale morphology of the transform fault spreading center intersections can be understood in terms of the transfer of shear fracture on a transform fault to tensile fracture on a spreading center propagating away from the intersection it is concluded that the transform fault must be weaker than the spreading center

late mesozoic and cenozoic stratigraphy and geological history of the south atlantic high latitudes abstract the complex analysis of various organic remains in upper mesozoic and cenozoic deposits of the southwest atlantic legs 36 40 71 and piston cores has led to a reliable subdivision of the sedimentary cover in its high latitudinal part planktonic foraminifers and calcareous nannoplankton dominate the biostratigraphy of the cretaceous and paleogene from the oligocene the leading role in the subdivision of sediments gradually passed to siliceous microfossils that provided an extremely detailed stratigraphy in the late cenozoic the history of oceanic subsidence sedimentation hiatuses and ccd fluctuations within the falkland plateau have been reconstructed on the basis of the biostratigraphic scheme obtained an attempt to trace cretaceous and cenozoic relative climatic fluctuations according to the changing in planktonic foraminiferal assemblages has been made oceanic and continental data combined have specified some aspects in the geological evolution of the high latitude south atlantic basin from its opening to date this paper emphasizes the importance of the circum antarctic current in paleoclimatic and sedimentational changes as well as the onset of the antarctic cover glaciation

a comparison of the isostatic response of bathymetric features in the north pacific ocean and philippine sea summary this paper concerns the calculation and analysis of admittance functions from large and uniform data sets of gravity and topography in four regions of the northern and western pacific ocean the purpose is to separate and describe possible differences in isostatic compensation between several type regions of oceanic crust a mid ocean ridge juan de fuca a mid plate seamount chain hawaiian ridge fracture zone topography on old crust north of hawaii and a marginal basin philippine sea results suggest that there are significant differences in the degree to which long wavelength topography has been compensated which can be distinguished between regions these differences are set in the perspective of three simple compensation mechanisms two of these consider local airy models in which raised topography is compensated at depth either by crustal roots or low density mantle a third considers the effects of an elastic plate of variable thickness supporting crustal variations conclusions are that a a thick plate possibly in excess of 30 km supports the hawaiian ridge b a much thinner plate of 5 to 15 km existed when the fracture zone topography was formed c the juan de fuca ridge is compensated either regionally by a plate 5 to 10 km thick or locally by sub crustal low densities at depths of 15 to 20 km and d the philippine sea shows no evidence for regional support ridges are compensated locally by differences in crustal thickness whereas the basins are underlain by density variations at depths comparable to those of the much younger juan de fuca ridge the major difference between admittance functions for the philippine sea and comparably aged regions of the north pacific ocean adds further new evidence of possible evolutionary differences between it and normal ocean basins