Myr In Age Research Articles

Discovery of radio jets in the Phoenix galaxy cluster center

Abstract We report the results of the Australia Telescope Compact Array (ATCA) 15 mm observation of the Phoenix galaxy cluster possessing an extreme star-burst brightest cluster galaxy (BCG) at the cluster center. We spatially resolved radio emission around the BCG, and found diffuse bipolar and bar-shape structures extending from the active galactic nucleus (AGN) of the BCG. They are likely radio jets/lobes, whose sizes are ∼10–20 kpc and locations are aligned with X-ray cavities. If we assume that the radio jets/lobes expand with the sound velocity, their ages are estimated to be ∼10 Myr. We also found compact radio emissions near the center and suggest that they are more young bipolar jets ∼1 Myr in age. Moreover, we found extended radio emission surrounding the AGN and discussed the possibility that the component is a product of the cooling flow, by considering synchrotron radiation partially absorbed by molecular clumps, free–free emission from the warm ionized gas, and the spinning dust emission from the dusty circumgalactic medium.

Late encounter events as source of disks and spiral structures

Context. Observations of arc-like structures and luminosity bursts of stars >1 Myr in age indicate that at least some stars undergo late infall events. Aims. We investigate scenarios of replenishing the mass reservoir around a star via capturing and infalling events of cloudlets. Methods. We carried out a total of 24 three-dimensional hydrodynamical simulations of cloudlet encounters with a Herbig star of mass 2.5 M⊙ using the moving-mesh code AREPO. To account for the two possibilities of a star or a cloudlet traveling through the interstellar medium (ISM), we put either the star or the cloudlet at rest with respect to the background gas. Results. For absent cooling in the adiabatic runs, almost none of the cloudlet gas is captured as a result of high thermal pressure. However, second-generation disks easily form when accounting for cooling of the gas. The disk radii range from several 100 to ~1000 au and associated arc-like structures up to 104 au in length form around the star for runs with and without stellar irradiation. Consistent with angular momentum conservation, the arcs and disks are larger for larger impact parameters. Accounting for turbulence in the cloudlet only mildly changes the model outcome. In the case of the star being at rest with the background gas, the disk formation and mass replenishment process is more pronounced and the associated arc-shaped streamers are longer lived. Conclusions. The results of our models confirm that late encounter events lead to the formation of transitional disks associated with arc-shaped structures such as observed for AB Aurigae or HD 100546. In addition, we find that second-generation disks and their associated filamentary arms are longer lived (>105 yr) in infall events, when the star is at rest with the background gas.

Late Cretaceous domatia reveal the antiquity of plant-mite mutualisms in flowering plants.

Mite houses, or acarodomatia, are found on the leaves of over 2000 living species of flowering plants today. These structures facilitate tri-trophic interactions between the host plant, its fungi or herbivore adversaries, and fungivorous or predaceous mites by providing shelter for the mite consumers. Previously, the oldest acarodomatia were described on a Cenozoic Era fossil leaf dating to 49 Myr in age. Here, we report the first occurrence of Mesozoic Era acarodomatia in the fossil record from leaves discovered in the Upper Cretaceous Kaiparowits Formation (76.6-74.5 Ma) in southern UT, USA. This discovery extends the origin of acarodomatia by greater than 25 Myr, and the antiquity of this plant-mite mutualism provides important constraints for the evolutionary history of acarodomatia on angiosperms.

A new compact young moving group around V1062 Scorpii

Aims.We are searching for new open clusters or moving groups in the solar neighbourhood.Methods.We used theGaia-TGAS catalogue, cut it into narrow proper motion and parallax slices and searched for significant spatial over-densities of stars in each slice. We then examined stars forming over-densities in optical and near-infrared colour-magnitude diagrams to determine if they are compatible with isochrones of a cluster.Results.We detected a hitherto unknown moving group or cluster in the Upper Centaurus Lupus (UCL) section of the Scorpius-Centaurus OB association (Sco-Cen) at a distance of 175 pc from the Sun. It is a group of 63 comoving stars of less than 10 to about 25 Myr in age. For the brightest stars that are present in theGaia-TGAS catalogue, the mean difference between kinematic and trigonometric distance moduli is − 0.01 mag with a standard deviation of 0.11 mag. Fainter cluster candidates are found in the HSOY catalogue, where no trigonometric parallaxes are available. For a subset of our candidate stars, we obtained radial velocity measurements at the MPG/ESO 2.2 m telescope in La Silla. Altogether we found 12 members with confirmed radial velocities and parallaxes, 31 with parallaxes or radial velocities, and 20 candidates from the convergent point method. The isochrone masses of our 63 members range from 2.6 to 0.7M⊙.

Chemical composition of evolved stars in the young open clusters NGC 4609 and NGC 5316

High-resolution spectral analysis is performed for the first time in evolved stars of two young open clusters: NGC 4609 and NGC 5316, of about 80 and 100 Myr in age, respectively, and turn-off masses above 5 Msun. Stellar evolution models predict an extra-mixing event in evolved stars, which follows the first dredge-up and happens later on the red giant branch. However, it is still not understood how this process affects stars of different masses. In this study, we determine abundances of the mixing sensitive elements carbon and nitrogen, carbon isotope 12C/13C ratios, as well as 20 other elements produced by different nucleosynthetic processes (O, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Y, Zr, Ba, La, Ce, Pr, Nd, and Eu). We compared our results with the latest theoretical models of evolutionary mixing processes. We find that the obtained 12C/13C and C/N ratios and [Na/Fe] agree quite well with the model which takes into account thermohaline- and rotation-induced mixing but within error limits also agree with the standard first dredge-up model. Comparison of oxygen, magnesium and other alpha-elements with theoretical models of Galactic chemical evolution revealed that both clusters follow the thin disk alpha-element trends. Neutron-capture element abundances in NGC 4609 are apparently reflecting its birthplace in the thin disk, while NGC 5316 has marginally higher abundances, which would indicate its birthplace in an environment more enriched with neutron-capture elements.

THE SPITZER SPACE TELESCOPE SURVEY OF THE ORION A AND B MOLECULAR CLOUDS. II. THE SPATIAL DISTRIBUTION AND DEMOGRAPHICS OF DUSTY YOUNG STELLAR OBJECTS

ABSTRACT We analyze the spatial distribution of dusty young stellar objects (YSOs) identified in the Spitzer Survey of the Orion Molecular clouds, augmenting these data with Chandra X-ray observations to correct for incompleteness in dense clustered regions. We also devise a scheme to correct for spatially varying incompleteness when X-ray data are not available. The local surface densities of the YSOs range from 1 pc−2 to over 10,000 pc−2, with protostars tending to be in higher density regions. This range of densities is similar to other surveyed molecular clouds with clusters, but broader than clouds without clusters. By identifying clusters and groups as continuous regions with surface densities ≥10 pc−2, we find that 59% of the YSOs are in the largest cluster, the Orion Nebula Cluster (ONC), while 13% of the YSOs are found in a distributed population. A lower fraction of protostars in the distributed population is evidence that it is somewhat older than the groups and clusters. An examination of the structural properties of the clusters and groups shows that the peak surface densities of the clusters increase approximately linearly with the number of members. Furthermore, all clusters with more than 70 members exhibit asymmetric and/or highly elongated structures. The ONC becomes azimuthally symmetric in the inner 0.1 pc, suggesting that the cluster is only ∼2 Myr in age. We find that the star formation efficiency (SFE) of the Orion B cloud is unusually low, and that the SFEs of individual groups and clusters are an order of magnitude higher than those of the clouds. Finally, we discuss the relationship between the young low mass stars in the Orion clouds and the Orion OB 1 association, and we determine upper limits to the fraction of disks that may be affected by UV radiation from OB stars or dynamical interactions in dense, clustered regions.

Sizes and shapes of young star cluster light profiles in M83

We measure the radii and two-dimensional light profiles of a large sample of young, massive star clusters in M83 using archival HST/WFC3 imaging of seven adjacent fields. We use GALFIT to fit the two-dimensional light profiles of the clusters, from which we find effective (half-light) radii, core radii, and slopes of the power-law (EFF) profile ($\eta$). We find lognormal distributions of effective radius and core radius, with medians of $\approx$2.5 pc and $\approx$1.3 pc, respectively. Our results provide strong evidence for a characteristic size of young, massive clusters. The average effective radius and core radius increase somewhat with cluster age. Little to no change in effective radius is observed with increasing galactocentric distance, except perhaps for clusters younger than 100 Myr. We find a shallow correlation between effective radius and mass for the full cluster sample, but a stronger correlation is present for clusters 200-300 Myr in age. Finally, the majority of the clusters are best fit by an EFF model with index $\eta\leq3.0$. There is no strong evidence for change in $\eta$ with cluster age, mass, or galactocentric distance. Our results suggest that clusters emerge from early evolution with similar radii and are not strongly affected by the tidal field of M83. Mass loss due to stellar evolution and/or GMC interactions appear to dominate cluster expansion in the age range we study.

THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. VI. THE RELIABILITY OF FAR-ULTRAVIOLET FLUX AS A STAR FORMATION TRACER ON SUBKILOPARSEC SCALES

We have used optical observations of resolved stars from the Panchromatic Hubble Andromeda Treasury (PHAT) to measure the recent (< 500 Myr) star formation histories (SFHs) of 33 FUV-bright regions in M31. The region areas ranged from ~$10^4$ to $10^6$ pc$^2$, which allowed us to test the reliability of FUV flux as a tracer of recent star formation on sub-kpc scales. The star formation rates (SFRs) derived from the extinction-corrected observed FUV fluxes were, on average, consistent with the 100-Myr mean SFRs of the SFHs to within the 1$\sigma$ scatter. Overall, the scatter was larger than the uncertainties in the SFRs and particularly evident among the smallest regions. The scatter was consistent with an even combination of discrete sampling of the initial mass function and high variability in the SFHs. This result demonstrates the importance of satisfying both the full-IMF and the constant-SFR assumptions for obtaining precise SFR estimates from FUV flux. Assuming a robust FUV extinction correction, we estimate that a factor of 2.5 uncertainty can be expected in FUV-based SFRs for regions smaller than $10^5$ pc$^2$, or a few hundred pc. We also examined ages and masses derived from UV flux under the common assumption that the regions are simple stellar populations (SSPs). The SFHs showed that most of the regions are not SSPs, and the age and mass estimates were correspondingly discrepant from the SFHs. For those regions with SSP-like SFHs, we found mean discrepancies of 10 Myr in age and a factor of 3 to 4 in mass. It was not possible to distinguish the SSP-like regions from the others based on integrated FUV flux.

Palaeomagnetic evidence for the neotectonic evolution of the Erzincan Basin, North Anatolian Fault Zone, Turkey

Ongoing motion of Anatolia towards the west is caused by convergence of the Arabian and Eurasian plates coupled with suction towards the retreating Hellenic Arc. This regime is controlling the development of neotectonic structures in Turkey with the resulting distributed deformation accommodated primarily between the East and North Anatolian Intracontinental Transform Faults. The Erzincan Basin is developed along the eastern part of the latter fault and although it incorporates one of the largest Quaternary basins in Turkey, the duration and tectonic evolution are disputed. Tectonic models proposed to explain the basin range from simple rhomboidal pull-apart to a complex multi-phase evolution. To help constrain the age and tectonic regime(s) forming the basin we have conducted a palaeomagnetic and geochronologic study of volcanic domes which occur mainly in proximity to strike-slip faulting along the northern margin of the basin. The investigated sample comprises 27 lava sites located within 14 cones, 13 to the north and one in the south. Although difficult propositions for palaeomagnetic investigation because the young predominantly-pyroclastic constructive topography is susceptible to collapse, all sites show positive inclinations and mainly northerly declinations showing that they are the consequence of a tectonic regime confined to the Brunhes Chron. Whilst the limitation of directional data from these young constructive features is stressed, ten cones are found to show clockwise rotations ranging from 12° to 195° with three cones showing no significant rotation. Geochronological studies from 13 samples yield a range of ages with 6 providing meaningful results <0.3Myr in age and consistent with young ages evident from morphology and paleomagnetism. AMS (Anisotropy of Magnetic Susceptibility) studies identify a fabric related to downslope flow at most sites with the majority moving away from conduits controlled by fractures paralleling the dominant NW-SE trend of the master fault. The palaeomagnetic and geochronologic results show that the history of the Erzincan Basin has involved at least two phases with the later phase incoporating an extensional component permitting access to mantle melts and confined to the last ∼300,000 years. The earlier phase commenced in Late Miocene or Early Pliocene times and initiated the rift infill which currently attains a maximum thickness in excess of 2.7km. We propose that the Erzincan Basin is now segmented as a mature basin by strike-slip cross faults although these cannot explain the consistent clockwise rotations observed within the small blocks incorporating the volcanic cones because these are confined to a narrow zone between two master faults and appear to be subject to ball-bearing style rotation.

Distribution of intact and core GDGTs in marine sediments

We conducted a survey of archaeal GDGT (glycerol dibiphytanyl glycerol tetraether) distributions in marine sediments deposited in a range of depositional settings. The focus was comparison of two pools presumed to have distinct geobiological significance, i.e. intact polar GDGTs (IP GDGTs) and core GDGTs (C GDGTs). The former pool has been suggested to be related to living communities of benthic archaea in marine sediments, while the latter is commonly interpreted to consist of molecular fossils from past planktonic archaeal communities that inhabited the surface ocean. Understanding the link between these two pools is important for assessment of the validity of current molecular proxies for sedimentary archaeal biomass and past sea surface temperatures. The relative distributions of GDGTs in the two pools in a core at a CH 4 rich site in the Black Sea provide evidence for in situ production of glycosidic IP GDGTs and their subsequent degradation to corresponding C GDGTs on timescales that are short in geological terms. In addition, we monitored the relationship between the IP GDGT and C GDGT pools in a sample set from various ocean basins with subseafloor depth from a few cm to 320 m and 0 to 4 Myr in age. Notable differences between the two pools can be summarized as follows: the GDGT with acyclic biphytanes, GDGT-0, and its analogues with two and three cyclopentane moieties (GDGT-2 and -3) are generally more abundant in the pool of IP GDGTs, while crenarchaeol tends to be more abundant in the C GDGT pool. Consequently, the ring index is generally higher for the C GDGTs while TEX 86, a molecular proxy ratio not considering the two major GDGTs, tends to be higher in the IP GDGT pool. These differences in the proportion of individual GDGTs in the two pools are probably due to in situ production of IP GDGTs with distributions differing from those of C GDGTs. Despite these differences, we observed significant correlation of these two ratios between the two pools. Specifically, in both pools TEX 86 is high in sediments from warm oceanic regimes and low in cold regimes. We discuss these relationships and suggest that recycling of core GDGTs by benthic archaea is an important mechanism linking both molecular pools.

EMPLOYING A NEW,BVIcPHOTOMETRIC SURVEY OF IC 4665 TO INVESTIGATE THE AGE OF THIS YOUNG OPEN CLUSTER

We present a new, BVIc photometric survey of the young open cluster IC4665, which improves on previous studies of this young cluster by incorporating a rigorous standardization procedure, thus providing high-fidelity colors and magnitudes for cluster members. We use this new photometric dataset to reevaluate the properties (age and distance) of IC4665. Namely, using a statistical approach incorporating Tau^2 CMD modeling, we measure a pre-main-sequence isochrone age and distance of 36+-9 Myr and 360+-12 pc, as well as a upper-main-sequence turn-off age and distance of 42+-12 Myr and 357+-12 pc. These ages and distances are highly dependent on the isochrone model and color used for the fitting procedure, with a possible range of ~10-20 Myr in age and ~20 pc in distance. This spread in calculated ages and distances seen between colors and models is likely due to limitations in the individual membership catalogs and/or systematic differences in the predicted stellar parameters from the different sets of models. Interestingly, when we compare the isochrone ages for IC4665 to the published lithium depletion boundary age, 28+-5 Myr, we observe that this cluster does not appear to follow the trend of isochrone ages being 1.5 times smaller than lithium depletion boundary ages. In addition, comparing the overall magnetic activity (X-ray and H-alpha emission) in IC4665 with other well studied open clusters, we find the observed activity distributions for this young cluster are best characterized by assuming an age of 30-40 Myr, thus in agreement with our pre-main-sequence and turn-off isochrone ages for IC4665. Overall, although some age discrepancies do exist, particularly in the ages measured from pre-main-sequence isochrones, the range of possible IC4665 ages derived from the various dating techniques employed here is relatively small compared to that found for other well studied open clusters.

SPITZERSPECTROSCOPY OF CIRCUMSTELLAR DISKS IN THE 5 Myr OLD UPPER SCORPIUS OB ASSOCIATION

We present mid-infrared spectra between 5.2 and 38 microns for 26 disk-bearing members of the ~5 Myr old Upper Scorpius OB association obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope. We find clear evidence for changes in the spectral characteristics of dust emission between the early (B+A) and late-type (K+M) infrared excess stars. The early-type members exhibit featureless continuum excesses that become apparent redward of ~8 microns. In contrast, 10 and 20 micron silicate features are present in all but one of the late-type excess members of Upper Scorpius. The strength of silicate emission among late-type Upper Scorpius members is spectral type dependent, with the most prominent features being associated with K5-M2 type stars. By fitting the spectral energy distributions (SED) of a sample of low-mass stars with accretion disk models, we find that the SEDs are consistent with models having inner disk radii ranging from ~0.2 to 1.2 AU. Complementary high resolution optical spectra for the Upper Scorpius excess stars were examined for signatures of gaseous accretion. Of the 35 infrared excess stars identified in Upper Scorpius, only 7 (all late-type) exhibit signatures of accretion. Mass accretion rates for these stars range from ~1e-11 to 1e-9 solar masses/yr. Compared to Class II sources in Taurus-Auriga, the disk population in Upper Scorpius exhibits reduced levels of near and mid-infrared excess emission and an order of magnitude lower mass accretion rates. These results suggest that the disk structure has changed significantly over the 2-4 Myr in age separating these two stellar populations. The ubiquity of depleted inner disks in the Upper Scorpius excess sample implies that such disks are a common evolutionary pathway that persists for some time.

Origins and diversification of Indo-West Pacific marine fauna: evolutionary history and biogeography of turban shells (Gastropoda, Turbinidae)

The present study aimed to assess the consequences of tectonic events and temperature regime on the diversification of Indo-West Pacific (IWP) turban shell species. Bayesian and parsimony methods were used to construct a phylogenetic hypothesis using sequence data from three genes for the turban shell genus Turbo and for a larger data set including representatives of all known genera in the subfamily Turbininae. Phylogenetic analyses indicate that all IWP Turbo species form a single clade approximately 68 Myr in age, predating the closure of the Tethys Sea and therefore predating the physical separation of the IWP from other biogeographical regions. All but one of the IWP subgenera tested in Turbo also predate the closure of the Tethys Sea. Fossil evidence for Marmarostoma, the largest subgenus, confirms that at least some Turbo lineages currently restricted to the IWP were previously more widespread. The combination of the phylogeny with the fossil evidence suggests that present day diversity in IWP Turbo is the result of the evolutionary persistence within the IWP of several, morphologically distinct lineages, some of which were more widespread in the Oligocene. Some IWP lineages show significant increases in diversification in the early Miocene, probably as a result of the increased availability of both shallow-water habitats due to tectonic plate movements and increased carbonate platforms in the central IWP resulting from coincident diversification of zooxanthellate corals. The IWP is therefore behaving as both a cradle of diversity (with new species originating in situ) and a museum of diversity (with lineages that predate its isolation also being maintained). Bayesian and parsimony analyses of the subfamily recovered five clades and mapping the temperature regime (tropical or temperate) of each species onto the molecular tree using parsimony suggested that temperate habitat is an ancestral character in at least four of the clades. This result was also supported by Bayesian reconstruction of ancestral states. Astralium (the fifth clade) may also prove to have a temperate origin, but this could not be determined with certainty given the available data. The origin of diversity in tropical regions is of particular interest because it has been suggested that the tropics are the source of many evolutionary novelties and have provided a species pool, from which temperate regions were populated. The present study suggests that Turbininae may be an exception to this rule. The tree shape also suggests that temperature has had an effect on speciation rates; temperate Turbininae are apparently evolving more slowly or suffering more extinction than their tropical sister clades, which show greater diversity.

The bark and ambrosia beetles (Curculionidae, Scolytinae) of Cocos Island, Costa Rica and the role of mating systems in island zoogeography

Cocos Island is a small oceanic island midway between Costa Rica and the Galápagos Archipelago; about 2 Myr in age, it is the only tropical oceanic island in the eastern Pacific with tropical wet forest. We identified several hundred bark beetle specimens collected during recent expeditions by INBio, the National Biodiversity Institute of Costa Rica, and re-examined all specimens from earlier collections. We report 19 species in ten genera, seven or eight of which are endemic, making scolytines the largest group of beetles known from the island. We describe as new Pycnarthrum pseudoinsulare, Xyleborinus cocoensis, and Xyleborus sparsegranulosus, resurrect Xyleborus bispinatus as separate from X. ferrugineus, and report six other species as new to Cocos Island. Three-quarters of the scolytines reproduce by brother–sister mating, and we argue that inbreeders are superior island colonists because they are less affected than are outbreeders by problems of mate location and inbreeding depression. The fauna and flora of Cocos Island arrived by dispersal and human transport. We examine natural colonization patterns for the fauna, using the distributions of the relatives of island endemics: most colonization came from the Americas, but the closest relatives to some endemics are found on Caribbean or Galápagos islands. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 89, 729–743.

Revised stratigraphy of Area 123, Koobi Fora, Kenya, and new age estimates of its fossil mammals, including hominins

Recent geologic study shows that all hominins and nearly all other published mammalian fossils from Paleontological Collection Area 123, Koobi Fora, Kenya, derive from levels between the KBS Tuff (1.87 ± 0.02 Ma) and the Lower Ileret Tuff (1.53 ± 0.01 Ma). More specifically, the fossils derive from 53 m of section below the Lower Ileret Tuff, an interval in which beds vary markedly laterally, especially those units containing molluscs and algal stromatolites. The upper Burgi Member (~2.00–1.87 Ma) crops out only in the southwestern part of Area 123. Adjacent Area 110 contains larger exposures of the member, and there the KBS Tuff is preserved as an airfall ash in lacustrine deposits and also as a fluvially redeposited ash. We observed no mammalian fossils in situ in this member in Area 123, but surface specimens have been documented in some monographic treatments. Fossil hominins from Area 123 were attributed to strata above the KBS Tuff in the 1970s, but later they were assigned to strata below the KBS Tuff (now called the upper Burgi Member). This study definitively places the Area 123 hominins in the KBS Member. Most of these hominins are between 1.60 and 1.65 myr in age, but the youngest may date to only 1.53 Ma, and the oldest, to 1.75 Ma. All are 0.15–0.30 myr younger than previously estimated. The new age estimates, in conjunction with published taxonomic attributions of fossils, suggest that at least two species of Homo coexisted in the region along with A. boisei until at least 1.65 Ma. Comparison of crania KNM-ER 1813 and KNM-ER 1470, which were believed to be of comparable age, is at the focus of the debate over whether Homo habilis sensu lato is in fact composed of two species: Homo habilis and Homo rudolfensis. These two crania are separated in time by ∼0.25 myr, and therefore, arguments for their conspecificity no longer need to confront the issue of unusually high contemporaneous variation within a single species.

Palaeoanthropological and molecular studies on the origin of modern humans in China

This article briefly discusses the antiquity of the first humans of China dating probably to 1.7 myr in age. Their origin was in Africa. Human evolution in China can be summarised as an hypothesis: ‘Continuity with Hybridization’ based on fossil evidence shown in Pleistocene human skulls sharing common features, the morphological mosaic between Homo erectus and Homo sapiens, and evidence indicating gene flow into and out of China. Molecular studies concerning the origin of modem humans in China indicate that the majority or the entire genetic source of the modern Chinese comes from Africa. These papers provide different ages for the most recent common ancestor of modern humans and that of non-Africans. Some of these are much longer than the ages suggested by most of previous molecular studies.

A detailed palaeomagnetic study of the oldest (≈15 Myr) lava sequences in Northwest Iceland

SUMMARY We have determined the directions of primary remanence in over 400 lava flows in the northwestern peninsula of Iceland (near 66.1°N, 23.3°W). These lavas were sampled in 16 profiles spread across 75 km, at the stratigraphic level of the oldest (approximately 15 Myr in age) of several distinctive lignite-bearing sediments in the peninsula. We find that the pattern of polarity reversals in the lava pile is broadly similar in these profiles but it is generally not possible to trace short polarity zones and excursions over more than a few kilometres laterally. Our results, which also include measurements on the lignite sediments at three sites, are consistent with previous suggestions that these sediment beds represent a time gap of the order of 0.2 Myr between the lavas above and below. The mean virtual geomagnetic pole of this collection is ‘far-sided’ by approximately 7°, and the scatter of individual poles is greater than that in palaeomagnetic surveys on younger lava series in Iceland.

Seismic velocities of the uppermost igneous crust versus age

SUMMARY Seismic refraction velocity data from the acoustic basement (called layer 2A) have been compiled for different mid-ocean ridges. The data from post-1970 studies show a strong correlation between velocity and basement age. Importantly, velocities double in less than 10 Myr for all ridges, but for older crust, up to 160 Myr in age, velocities do not increase significantly.

Observation of sections of oceanic crust and mantle cropping out on the southern wall of Kane FZ (N. Atlantic)

ABSTRACTThe objective of the 20 Nautile dives of the recent Kanaut cruise was to study the southern wall of the Kane Fracture Zone from its eastern intersection with the Mid‐Atlantic Ridge (MAR) to 5 Myr in age. The geological mapping shows four successive massifs, wrench faulted and slightly tilted. The transform‐facing walls of these massifs exhibit outcrops of fresh and serpentinized peridotites, gabbros and basalts. The entire crustal exposure is cataclased and metamorphosed to greenschist facies.

The anisotropic structure of the upper mantle in the Pacific

Anisotropic inversions of surface wave data show that the variations in vertical shear velocity, βv, and anisotropy of the oceanic upper mantle in the Pacific are much smoother and more systematic functions of the age of the seafloor than has been reported in previous studies. The data used in this analysis are the pure-path results of previous studies on the lateral distribution of fundamental-mode Love and Rayleigh wave phase velocities (Nishimura & Forsyth 1985, 1988). The pure-path models include parameters which describe the variations with age, azimuthal anisotropy and residual depth anomalies. The calculated velocity models of the upper mantle are constrained to vary smoothly with depth and to represent the minimum deviation from an isotropic starting model. Inversions were performed using the method of Tarantola & Valette (1982). The two best resolved parameters of the computed transversely isotropic model are the shear wave velocity terms, βv and ξ. The results indicate that βv above 200 km progressively increases as a function of the age of the seafloor with the pattern qualitatively mimicking isotherms of theoretical thermal cooling models. If one selects the depth to the maximum negative gradient in shear velocity as being the best available indicator of lithospheric thickness, then the thickness increases from about 15–35 km beneath 0–4 Myr old seafloor to 70–110 km in the oldest seafloor. The magnitude of the shear wave anisotropy term, ξ, rapidly increases in the first 20 Myr until some apparent constant value is reached in the older regions. A more realistic upper mantle structure is calculated using a priori information on the correlation between changes in shear and compressional wave velocities and the expected nature of the anisotropy. The general results are the same as the previous inversion without a priori constraints. Finally, the effect of attenuation is included, the primary result being an overall increase in βv. The maximum change occurs at around 150 km depth, which reduces the velocity contrast between the lithosphere and asthenosphere. It is therefore more difficult to make a distinction between the plate and low-velocity zone when the effect of attenuation is included. An estimate of the azimuthal anisotropic structure is obtained by inverting for the Rayleigh wave cos 2ψ coefficients using derivatives calculated by the method of Montagner & Nataf (1986). The reference frame used to constrain the azimuthal effect is that of fossil seafloor spreading direction. The results indicate that in regions of the Pacific less than 80 Myr in age, there is significant anisotropy down to 200 km depth. In regions older than 80 Myr, azimuthal anisotropy is confined to the upper 50 km. The transverse and azimuthal anisotropy structures can be explained by an oceanic upper mantle containing olivine with different orientations.