Structure of the Malpelo Ridge (Colombia) from seismic and gravity modelling

Wide-angle and multichannel seismic data collected on the Malpelo Ridge provide an image of the deep structure of the ridge and new insights on its emplacement and tectonic history. The crustal structure of the Malpelo Ridge shows a 14 km thick asymmetric crustal root with a smooth transition to the oceanic basin southeastward, whereas the transition is abrupt beneath its northwestern flank. Crustal thickening is mainly related to the thickening of the lower crust, which exhibits velocities from 6.5 to 7.4 km/s. The deep structure is consistent with emplacement at an active spreading axis under a hotspot like the present-day Galapagos Hotspot on the Cocos-Nazca Spreading Centre. Our results favour the hypothesis that the Malpelo Ridge was formerly a continuation of the Cocos Ridge, emplaced simultaneously with the Carnegie Ridge at the Cocos-Nazca Spreading Centre, from which it was separated and subsequently drifted southward relative to the Cocos Ridge due to differential motion along the dextral strike-slip Panama Fracture Zone. The steep faulted northern flank of the Malpelo Ridge and the counterpart steep and faulted southern flank of Regina Ridge are possibly related to a rifting phase that resulted in the Coiba Microplate’s separation from the Nazca Plate along the Sandra Rift.     

From oblique subduction to intra-continental transpression: Structures of the southern Kermadec-Hikurangi margin from multibeam bathymetry,side-scan sonar and seismic reflection

The southern Kermadec-Hikurangi convergent margin, east of New Zealand, accommodates the oblique subduction of the oceanic Hikurangi Plateau at rates of 4–5 cm/yr. Swath bathymetry and sidescan data, together with seismic reflection and geopotential data obtained during the GEODYNZ-SUD cruise, showed major changes in tectonic style along the margin. The changes reflect the size and abundance of seamounts on the subducting plateau, the presence and thickness of trench-fill turbidites, and the change to increasing obliquity and intracontinental transpression towards the south. In this paper, we provide evidence that faulting with a significant strike-slip component is widespread along the entire 1000 km margin. Subduction of the northeastern scrap of the Hikurangi Plateau is marked by an offset in the Kermadec Trench and adjacent margin, and by a major NW-trending tear fault in the scarp. To the south, the southern Kermadec Trench is devoid of turbidite fill and the adjacent margin is characterized by an up to 1200 m high scarp that locally separates apparent clockwise rotated blocks on the upper slope from strike-slip faults and mass wasting on the lower slope. The northern Hikurangi Trough has at least 1 km of trench-fill but its adjacent margin is characterized by tectonic erosion. The toe of the margin is indented by 10–25 km for more than 200 km, and this is inferred to be the result of repeated impacts of the large seamounts that are abundant on the northern Hikurangi Plateau. The two most recent impacts have left major indentations in the margin. The central Hikurangi margin is characterized by development of a wide accretionary wedge on the lower slope, and by transpression of presubduction passive margin sediments on the upper slope. Shortening across the wedge together with a component of strike-slip motion on the upper slope supports an interpretation of some strain partitioning. The southern Hikurangi margin is a narrow, mainly compressive belt along a very oblique, apparently locked subduction zone.     

Alignment Trends of Velocity Gradients and Subgrid-Scale Fluxes in the Turbulent Atmospheric Boundary Layer

Field experimental data in the atmospheric surface layer are analyzed using toolsfrom statistical geometry. The data consist of velocity measurements from sonicanemometer arrays. In the context of large eddy simulations (LES), these arrayspermit the spatial filtering needed to separate large from small scales. Time seriesof various quantities relevant to LES are evaluated from the data. Results show thatthe preferred filtered fluid deformation is axisymmetric extension and the preferredsubgrid stress state is axisymmetric contraction. The filtered fluctuating vorticityshows preferred alignments with the mean vorticity, with the streamwise direction,and with the intermediate strain-rate eigenvector. The alignment between eigenvectorsof the subgrid-scale stress and filtered strain rate is used to test eddy viscosity andmixed model formulations. In qualitative agreement with prior laboratory measurements at much lower Reynolds numbers, a bimodal distribution is observed, which can be reduced to good alignment with eddy viscosity closure using the mixed model.     

Mineralogical and isotopic properties of inorganic nanocrystalline magnetites

Inorganic magnetite nanocrystals were synthesized in an aqueous medium at 25°C, atmospheric pressure, ionic strength of 0.1 M, oxygen fugacity close to 0, and under controlled chemical affinity, which was maintained constant during an experiment and varied between different experiments. The total concentration of iron in the initial solutions, with Fe(III)/Fe(II) ratios of 2, was varied in order to measure the role of this parameter on the reaction rate, particle morphology, and oxygen isotopic composition. The reaction rates were followed by a pHstat apparatus. The nature and morphology of particles were studied by transmission electron microscopy and electron energy loss spectroscopy. Fractionation factors of oxygen isotopes were determined by mass spectrometry after oxygen extraction from the solid on BrF₅ lines. At low total iron concentrations, goethite and poorly crystalline iron oxides were observed coexisting with magnetite. At higher concentrations, euhedral single crystals of pure magnetite with an average characteristic size of 10 nm were formed, based on a first-order rate law with respect to total iron concentration. These results confirm that, under high supersaturation conditions, low-temperature inorganic processes can lead to the formation of well-crystallized nanometric magnetite crystals with narrow size distribution. The observed oxygen isotope fractionation factor between magnetite crystals and water was of 0-1‰, similar to the fractionation factor associated with bacterially produced magnetite. We suggest that the solution chemistry used in this study for inorganic precipitation is relevant to better understanding of magnetite precipitation in bacterial magnetosomes, which might thus be characterized by high saturation states and pH.     

Determination of cirrus radiative parameters from combination between active and passive remote sensing measurements during FRENCH/DIRAC 2001

In the context of the next AQUA Train satellite experiment, airborne measurements were carried out to simulate satellite measurements. They were conducted between September 25 and October 12, 2001, off the coast of southern France over the Atlantic Ocean and over the Mediterranean Sea, respectively. During the intensive Field Radiation Experiment on Natural Cirrus and High-level clouds (FRENCH/DIRAC 2001), natural ice clouds were sampled from in situ and remote sensing measurements. On October 5 and 7, 2001, cirrus cloud decks were described by a complete data set acquired by: (i) in situ microphysical instruments onboard the TBM-700 aircraft: PMS probe, and Polar Nephelometer (ii) and downward-looking radiative instruments onboard the Mystère 20 aircraft: an infrared radiometer, a lidar, a visible imager with polarisation capabilities, and a middle infrared radiometer. Moreover, classical thermodynamical measurements were carried out onboard the Mystère 20. Mean microphysical characteristics of cirrus deck are derived from interpretation of remote sensing measurements. These properties are compared with those derived from in situ microphysical measurements in order to evaluate the radiative impact of natural cirrus clouds.     

Measurement of fluid contents and wetting front profiles by real-time neutron radiography

Few methods exist for measuring rapidly changing fluid contents at the pore scale that simultaneously allow whole flow field visualization. We present a method for using real-time neutron radiography to measure rapidly changing moisture profiles in porous media. The imaging technique monitors the attenuation of a thermal neutron beam as it traverses a flow field and provides measurements every 30 ms with an image area >410 cm² and a spatial resolution 0.05 cm. The technique is illustrated by measuring the variation in moisture content across a wetting front moving at constant velocity through SiO₂ sand. The relative contributions of the hydraulic conductivity and diffusivity terms in Richards' equation to the total fluid flux within the wetting front region were also measured. The diffusivity was found to rise from zero to a peak value within the wetting front region before falling off while the conductivity was found to rise monotonically. The reliability of the technique was checked via mass balance.