From rifting to spreading in the eastern Gulf of Aden: a geophysical survey of a young oceanic basin from margin to margin

A geophysical survey in the eastern Gulf of Aden, between the Alula–Fartak (52°E) and the Socotra (55°E) transform faults, was carried out during the Encens–Sheba cruise. The conjugate margins of the Gulf are steep, narrow and asymmetric. Asymmetry of the rifting process is highlighted by the conjugate margins (horst and graben in the north and deep basin in the south). Two transfer fault zones separate the margins into three segments, whereas the present‐day Sheba Ridge is divided into two segments by a transform discontinuity. Therefore segmentation of the Sheba Ridge and that of the conjugate margins did coincide during the early stages of oceanic spreading. Extensive magma production is evidenced in the central part of the western segment. Anomaly 5d was identified in the northern and southern parts of the oceanic basin, thus confirming that seafloor spreading in this part of Gulf of Aden started at least 17.6 Ma ago.     

Discovery of a double impact crater in Libya: the astrobleme of Arkenu

Using orbital imaging radar, we detected a double circular structure, located in the southeastern part of the Libyan Desert, which is partially hidden under sandy sediments. Fieldwork confirmed it to be an unknown double impact crater, each crater having a diameter of about 10 km, younger than 140 Ma. Sampling on the site enabled the observation of quantities of shatter cone structures and impact breccias containing planar fractures. To cite this article: P. Paillou et al., C. R. Geoscience 335 (2003).     

On the variation of pulsar periods in close binary systems

X-ray pulsars, which form together with a main sequence star or a giant a close binary system, show strong variations in their pulse period. Epochs of spin-up and spin-down interchange on timescales of some tens up to some thousand days.

We study this phenomenon in the framework of the disk-fed model by Ghosh and Lamb, which we generalize by allowing for an inclination angle χ between the magnetic dipole moment μ and the rotation axis. Moreover, we take into account the relatively small magnetic field induced by currents in the magnetosphere flowing along the dipole field lines. This component, which we calculate numerically, leads to a torque perpendicular to the rotation axis and induces a precession of it. Due to this additional degree of freedom, the calculated pulse period history is much smoother than in the classical Ghosh and Lamb model. Within this extended model we compute, as an example, the pulse period history of Cen X-3 using the observed X-ray flux as a measure for the mass accretion rate. Qualitative accordance with the measured pulse period data (see e.g. Nelson et al., 1997 [ApJ, 488, L117]) is found to be good.     

Evidence of Organized Large Eddies by Ground-Based Doppler Lidar, Sonic Anemometer and Sodar

In an experiment investigating the planetary boundary layer (PBL) wind and temperature fields, and PBL inversion height recorded by various instruments, the results reveal the presence of organized large eddies (OLE) or rolls. The measurements by lidars, anemometers, soundings and sodar gave an overview of the characteristics of the rolls and sources of energy production that maintain them. The experimental results obtained on two consecutive days are compared to model outputs. The agreement is excellent, showing that thermal stratification and wind shear are important factors in the structure and dynamics of OLE. A heterodyne Doppler lidar (HDL) is shown to be a useful tool in the study of OLE.     

Turbulent viscosity measurements relevant to planetary core-mantle dynamics

Laboratory experiments that combine thermal convection in a rapidly rotating shell with a sudden increase of the shell’s rotation rate (spin-up) enable us to study processes related to turbulent viscous coupling between planetary fluid cores and solid mantles. We experimentally measure the large-scale effective viscosity by determining how the synchronisation time between the fluid and the shell (called the spin-up time) is shortened when convective turbulence exists in the bulk of the fluid. Our experiments suggest that viscous core-mantle coupling in planets may be greater than has been previously estimated using molecular viscosity values.     

Attenuation characteristics of peak ground acceleration from fault trace of the 1999 Kocaeli (Turkey) earthquake and comparison of spectral acceleration with seismic design code

The 17 August 1999 Kocaeli earthquake in Turkey produced a majorsurface rupture. We traced this surface rupture from Gölcük toDüzce and located it accurately by using GPS. The closest distancefrom the surface rupture to the strong motion observation sites weredetermined. Then the attenuation characteristics of the observed peakground acceleration were compared with the attenuation relation given byFukushima and Tanaka (1992), which is suitable for the near-fault zone inJapan and gives results that closely match data recorded during the 1995Hyogo-ken Nanbu earthquake in Japan. Although this attenuation relationwas developed for Japan, we found that it agreed well with the KOCAELIearthquake. Furthermore, the observed spectral acceleration of 5%damping was compared with the building design code of Turkey and theobserved level was lower than the code.     

Relationships among Gibbs free energies and enthalpies of formation of phosphates,oxides and aqueous ions

Two parameters _GO^2– and _HO^2– are defined as the differences between respectively the Gibbs free energies and the enthalpies of formation of an oxide and its corresponding aqueous cation. The Gibbs free energies and enthalpies of formation of phosphates from their consituent oxides are shown to be linear functions of respectively _GO^2– and _HO^2– of their constituent cations.