Spatial and temporal variations in crustal production at the Mid-Atlantic Ridge, 25°N–27°30′N and 0–27 Ma

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 120 (2015): 2119–2142, doi:10.1002/2014JB011501.We use high-resolution multibeam bathymetry, shipboard gravity, side-scan sonar images, and magnetic anomaly data collected on conjugate flanks of the Mid-Atlantic Ridge at 25°N–27°30′N and out to ~27 Ma crust to investigate the crustal evolution of the ridge. Substantial variations in crustal structure and thickness are observed both along and across isochrons. Along isochrons within spreading segments, there are distinct differences in seafloor morphology and gravity-derived crustal thickness between inside and outside corners. Inside corners are associated with shallow depths, thin crust, and enhanced normal faulting while outside corners have greater depths, thicker crust, and more limited faulting. Across-isochrons, systematic variations in crustal thickness are observed at two different timescales, one at ~2–3 Myr and another at >10 Myr, and these are attributed to temporal changes in melt supply at the ridge axis. The shorter-term variations mostly are in-phase between conjugate ridge flanks, although the actual crustal thickness can be significantly different on the two flanks at any given time. We observe no correlation between crustal thickness and spreading rate. Thus, during periods of low melt supply, tectonic extension must increase to accommodate the full plate separation rate. This extension commonly is concentrated in long-lived faults on only one side of the axial valley, resulting in strong across-axis asymmetries in crustal thickness and seafloor morphology. The thin-crust flank has few volcanic features and exhibits elevated, blocky topography with large-offset, often irregular faults, while the conjugate thicker-crust flank shows shorter-offset, regular faulting, and common volcanic features. The variations in melt supply at the ridge axis most likely are caused either by episodic convection in the subaxial mantle or by variable melting of chemically heterogeneous mantle.This study was funded by Chinese Natural Science Foundation grant 41206034 and Chinese Postdoc Scholarship award 2012M511130 (T.W.), by Ministry of Science and Technology 973 Project award 2012CB417303, and by the WHOI Henry Bryant Bigelow Chair (J.L.). ARSRP and MAREAST data acquisition was funded by Office of Naval Research grant N00014-90-J-6121 and by U.S. National Science Foundation grant OCE-9503561, respectively.2015-10-2

Cemented mounds and hydrothermal sediments on the detachment surface at Kane Megamullion : a new manifestation of hydrothermal venting

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 14 (2013): 3352–3378, doi:10.1002/ggge.20186.Long-lived detachment faults are now known to be important in tectonic evolution of slow-spreading mid-ocean ridges, and there is increasing evidence that fluid flow plays a critical role in development of detachment systems. Here we document a new manifestation of low-temperature hydrothermal venting associated with the detachment fault that formed Kane Megamullion ∼3.3–2.1 m.y. ago in the western rift-valley wall of the Mid-Atlantic Ridge. Hydrothermal effects on the detachment surface include (1) cemented mounds of igneous rock and chalk debris containing hydrothermal Mn oxides and Fe oxyhydroxides, and (2) layered deposits of similar Fe-Mn minerals ± interbedded chalks. Mounds are roughly conical, ∼1–10 m high, and contain primarily basalts with lesser gabbro, serpentinite, and polymict breccia. The layered Fe-Mn-rich sediments are flat-bedded to contorted and locally are buckled into low-relief linear or polygonal ridges. We propose that the mounds formed where hydrothermal fluids discharged through the detachment hanging wall near the active fault trace. Hydrothermal precipitates cemented hanging-wall debris and welded it to the footwall, and this debris persisted as mounds as the footwall was exhumed and surrounding unconsolidated material sloughed off the sloping detachment surface. Some of the layered Fe-Mn-rich deposits may have precipitated from fluids discharging from the hanging-wall vents, but they also precipitated from low-temperature fluids venting from the exposed footwall through overlying chalks. Observed natural disturbance and abnormally thin hydrogenous Fe-Mn crusts on some contorted, hydrothermal Fe-Mn-rich chalks on ∼2.7 Ma crust suggest diffuse venting that is geologically recent. Results of this study imply that there are significant fluid pathways through all parts of detachment systems and that low-temperature venting through fractured detachment footwalls may continue for several million years off-axis.NSF grant 0118445 supported data acquisition and processing for Knorr Cruise 180- 2. The Deep Ocean Exploration Institute at Woods Hole Oceanographic Institution supported research and analytical costs for this study.2014-03-0

Benthic storms, nepheloid layers, and linkage with upper ocean dynamics in the western North Atlantic

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Marine Geology 385 (2017): 304–327, doi:10.1016/j.margeo.2016.12.012.Benthic storms are episodic periods of strong abyssal currents and intense, benthic nepheloid (turbid) layer development. In order to interpret the driving forces that create and sustain these storms, we synthesize measurements of deep ocean currents, nephelometer-based particulate matter (PM) concentrations, and seafloor time-series photographs collected during several science programs that spanned two decades in the western North Atlantic. Benthic storms occurred in areas with high sea-surface eddy kinetic energy, and they most frequently occurred beneath the meandering Gulf Stream or its associated rings, which generate deep cyclones, anticyclones, and/or topographic waves; these create currents with sufficient bed-shear stress to erode and resuspend sediment, thus initiating or enhancing benthic storms. Occasionally, strong currents do not correspond with large increases in PM concentrations, suggesting that easily erodible sediment was previously swept away. Periods of moderate to low currents associated with high PM concentrations are also observed; these are interpreted as advection of PM delivered as storm tails from distal storm events. Outside of areas with high surface and deep eddy kinetic energy, benthic nepheloid layers are weak to non-existent, indicating that benthic storms are necessary to create and maintain strong nepheloid layers. Origins and intensities of benthic storms are best identified using a combination of time-series measurements of bottom currents, PM concentration, and bottom photographs, and these should be coupled with water-column and surface-circulation data to better interpret the specific relations between shallow and deep circulation patterns. Understanding the generation of benthic nepheloid layers is necessary in order to properly interpret PM distribution and its influence on global biogeochemistry.Funding for construction of the Bottom Ocean Monitor was provided by Lamont-Doherty Geological Observatory (now Lamont-Doherty Earth Observatory). BOM and mooring deployments and data analysis were funded by the Office of Naval Research (contracts N00014-75-C-0210 and N00014-80-C-0098 to Biscaye and Gardner at Lamont-Doherty; Contracts N00014-79-C-0071 and N00014-82-C-0019 at Woods Hole Oceanographic Institution and ONR Contracts N00014-75-C-0210 and N00014-80-C-0098 at Lamont-Doherty Geological Observatory to Tucholke), Sandia National Laboratories (contract SL-16-5279 to Gardner), the National Science Foundation (contract OCE 1536565 to Gardner and Richardson), Earl F. Cook Professorship (Gardner), and the Department of Energy (contract DE-FG02-87ER-60555 to Biscaye)

UIT Detection of Hot Stars in the Globular Cluster NGC362

We used the Ultraviolet Imaging Telescope during the March 1995 Astro-2 mission to obtain a deep far-UV image of the globular cluster NGC 362, which was formerly thought to have an almost entirely red horizontal branch (HB). 84 hot (T_eff > 8500 K) stars were detected within a radius of 8'.25 of the cluster center. Of these, 43 have FUV magnitudes consistent with HB stars in NGC 362, and at least 34 are cluster members. The number of cluster members is made uncertain by background contamination from blue stars in the Small Magellanic Cloud (SMC). There are six candidate supra-HB stars which have probably evolved from the HB. We discuss the implications of these results for the production of hot blue stars in stellar populations.Comment: 10 pages AASLaTeX including one postscript figure and one compressed bitmap, .jpg format. To appear in Ap. J. Letters. Postscript version also available at http://www.astro.virginia.edu/~bd4r

The Clusters AgeS Experiment (CASE). II. The Eclipsing Blue Straggler OGLEGC-228 in the Globular Cluster 47 Tuc

We use photometric and spectroscopic observations of the eclipsing binary OGLEGC-228 (V228) to derive the masses, radii, and luminosities of the component stars. Based on measured systemic velocity, proper motion and distance, the system is a blue straggler member of the globular cluster 47 Tuc. Our analysis shows that V228 is a semi-detached Algol. We obtain M=1.512 +/- 0.022 Msun, R=1.357 +/- 0.019 Rsun, L=7.02 +/- 0.050 Lsun for the hotter and more luminous primary component and M=0.200 +/- 0.007 Msun, R=1.238 +/- 0.013 Rsun, L=1.57 +/- 0.09 Lsun for the Roche lobe filling secondary.Comment: 19 pages, 5 figures, AJ, in pres

Sedimentation rates test models of oceanic detachment faulting

This is the accepted manuscript version.The final version is available from Wiley at http://onlinelibrary.wiley.com/doi/10.1002/2014GL061555/full.Long-lived detachment faults play an important role in the construction of new oceanic crust at slow-spreading mid-oceanic ridges. Although the corrugated surfaces of exposed low-angle faults demonstrate past slip, it is difficult to determine whether a given fault is currently active. If inactive, it is unclear when slip ceased. This judgment is crucial for tectonic reconstructions where detachment faults are present, and for models of plate spreading. We quantify variation in sediment thickness over two corrugated surfaces near 16.5°N at the Mid-Atlantic Ridge using near-bottom CHIRP data. We show that the distribution of sediment and tectonic features at one detachment fault is consistent with slip occurring today. In contrast, another corrugated surface 20 km to the south shows a sediment distribution suggesting that slip ceased ~150,000 years ago. Data presented here provide new evidence for active detachment faulting, and suggest along-axis variations in fault activity occur over tens of kilometers.This work was supported by the National Science Foundation grant number OCE-1155650

Early-type objects in NGC6611 and Eagle Nebula

An important question about Be stars is whether Be stars are born as Be stars or not. It is necessary to observe young clusters to answer this question. Observations of stars in NGC6611 and the star-formation region of Eagle Nebula have been carried out with the ESO-WFI in slitless spectroscopic mode and at the VLT-GIRAFFE. The targets for the GIRAFFE observations were pre-selected from the literature and our catalogue of emission-line stars (ELS) based on the WFI study. GIRAFFE observations allowed us to study accurately the population of the early-type stars with and without emission lines. For this study, we determined the fundamental parameters of OBA stars. We also studied the status of the objects (main sequence or pre-main sequence stars) by using IR data, membership probabilities, and location in HR diagrams. The nature of the early-type ELS in M16 is derived. The slitless observations with the WFI clearly indicate a small number of ELS in M16. We observed with GIRAFFE 101 OBA stars, among them 9 are ELS with circumstellar emission in Halpha. We found that: W080 could be a new He-strong star, like W601. W301 is a possible classical Be star, W503 is a mass-transfer eclipsing binary with an accretion disk, and the other ones are possible Herbig Ae/Be stars. We also found that the rotational velocities of main sequence B stars are 18% lower than those of pre-main sequence B stars, in good agreement with theory about the evolution of rotational velocities. Combining different indications and technics, we found that 27% of the B-type stars are binaries. We also redetermined the age of NGC6611 found equal to 1.2--1.8 Myears in good agreement with the most recent determinations.Comment: Accepted by A&A, english not yet correcte

Mylonitic deformation at the Kane oceanic core complex : implications for the rheological behavior of oceanic detachment faults

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 14 (2013): 3085–3108, doi:10.1002/ggge.20184.The depth extent, strength, and composition of oceanic detachment faults remain poorly understood because the grade of deformation-related fabrics varies widely among sampled oceanic core complexes (OCCs). We address this issue by analyzing fault rocks collected from the Kane oceanic core complex at 23°30′N on the Mid-Atlantic Ridge. A portion of the sample suite was collected from a younger fault scarp that cuts the detachment surface and exposes the interior of the most prominent dome. The style of deformation was assessed as a function of proximity to the detachment surface, revealing a ∼450 m thick zone of high-temperature mylonitization overprinted by a ∼200 m thick zone of brittle deformation. Geothermometry of deformed gabbros demonstrates that crystal-plastic deformation occurred at temperatures >700°C. Analysis of the morphology of the complex in conjunction with recent thermochronology suggests that deformation initiated at depths of ∼7 km. Thus we suggest the detachment system extended into or below the brittle-plastic transition (BPT). Microstructural evidence suggests that gabbros and peridotites with high-temperature fabrics were dominantly deforming by dislocation-accommodated processes and diffusion creep. Recrystallized grain size piezometry yields differential stresses consistent with those predicted by dry-plagioclase flow laws. The temperature and stress at the BPT determined from laboratory-derived constitutive models agree well with the lowest temperatures and highest stresses estimated from gabbro mylonites. We suggest that the variation in abundance of mylonites among oceanic core complexes can be explained by variation in the depth of the BPT, which depends to a first order on the thermal structure and water content of newly forming oceanic lithosphere.Knorr Cruise 180-2 data and sample acquisition was supported by NSF grant 0118445.2014-02-2

Morphology and tectonics of the Mid-Atlantic Ridge, 7°–12°S

We present swath bathymetric, gravity, and magnetic data from the Mid-Atlantic Ridge between the Ascension and the Bode Verde fracture zones, where significant ridge–hot spot interaction has been inferred. The ridge axis in this region may be divided into four segments. The central two segments exhibit rifted axial highs, while the northernmost and southernmost segments have deep rift valleys typical of slow-spreading mid-ocean ridges. Bathymetric and magnetic data indicate that both central segments have experienced ridge jumps since ~1 Ma. Mantle Bouguer anomalies (MBAs) derived from shipboard free air gravity and swath bathymetric data show deep subcircular lows centered on the new ridge axes, suggesting that mantle flow has been established beneath the new spreading centers for at least ~1 Myr. Inversion of gravity data indicates that crustal thicknesses vary by ~4 km along axis, with the thickest crust occurring beneath a large axial volcanic edifice. Once the effects of lithospheric aging have been removed, a model in which gravity variations are attributed entirely to crustal thickness variations is more consistent with data from an axis-parallel seismic line than a model that includes additional along-axis variations in mantle temperature. Both geophysical and geochemical data from the region may be explained by the melting of small (<200 km) mantle chemical heterogeneities rather than elevated temperatures. Therefore, there may be no Ascension/Circe plume

Origin and geochemical evolution of the Madeira-Tore Rise (eastern North Atlantic)

The Madeira-Tore Rise, located ∼700 km off the NW African coast, forms a prominent ridge in the east Atlantic. The age and origin of the rise are controversial. This study presents major and trace element, Sr, Nd, Pb, Hf isotope and 40Ar/39Ar age determinations from volcanic rocks dredged from different sites along the rise. In addition, isotopic compositions of rock samples from Great Meteor Seamount in the central Atlantic are presented. The new radiometric and paleontologically constrained ages identify two major episodes of volcanism: The first is the base of the rise (circa 80 to >95 Ma) and the second is seamounts on the rise (0.5–16 Ma). It is proposed that interaction of the Canary hot spot with the Mid-Atlantic spreading center formed the deep basement of the Madeira-Tore Rise and the J-Anomaly Ridge west of the Atlantic spreading center in the Mid-Cretaceous. Age and geochemical data and plate tectonic reconstructions suggest, however, that the recovered Late Cretaceous volcanic rocks represent late stage volcanism from the time when the Madeira-Tore Rise was still close to the Canary hot spot. Long after moving away from the influence of the Canary hot spot, the Madeira-Tore Rise was overprinted by late Cenozoic volcanism. Miocene to Pleistocene volcanism at the northern end of the rise can be best explained by decompression mantle melting beneath extensional sectors of the Azores-Gibraltar Fracture Zone (African-Eurasian plate boundary). The geochemical compositions of these volcanic rocks suggest that the magmas were variably contaminated by enriched material within or derived by melting of enriched material underplated at the base of the lithosphere, possibly originating from the Cretaceous Canary plume. Alternatively, these late Cenozoic volcanic rocks may have derived from decompression melting of enriched pyroxenitic/eclogitic material in the upper mantle. Isotopically more depleted Pliocene to Pleistocene volcanism at the southern end of the Madeira-Tore Rise may be related to the nearby Madeira hot spot