Solar Coronal Plasma in Doppler Measurements

Doppler tracking of an interplanetary spacecraft near solar conjunction is strongly affected by the plasma in the solar corona, the main competitive contribution in measurements of the gravitational deflection of light rays. With the simultaneous availability of carriers in X band and K_a band for interplanetary communications, the plasma contribution to the corona can be accurately eliminated and measured. If, as in the Cassini mission, three different observables are available, this can be done in two ways: one deals with the total plasma content in the electric approximation, even in the ionosphere and interplanetary space; another is limited to the corona, but has access to subtler effects, like the magnetic correction to the refractive index. This technique will allow important progress in the radio investigation of the solar corona.     

Unexpected Jurassic to Neogene vertical movements in ‘stable’ parts of NW Africa revealed by low temperature geochronology

In Morocco, it is generally considered that post‐Hercynian vertical movements were limited to the Atlas system, the passive continental margin and the Rif. Apatite FT and He ages from the Moroccan Meseta (Rehamna and Zaer Massif) document instead two episodes of subsidence and exhumation in Jurassic‐Early Cretaceous and during the Late Cretaceous to Neogene. The Meseta subsided to >3 km depth during the Late Triassic to Middle Jurassic and was exhumed to the surface before the Late Cretaceous, during the rift and post‐rift stages of Central Atlantic opening. Erosion of the exhuming rocks is responsible for a thick package of terrigenous sands found in the Moroccan offshore and elsewhere along the NW Africa margin. About 1 km of subsidence affected the Meseta during the Late Cretaceous to Eocene. During the Neogene, these areas were brought back to the surface in association with bimodal folding with wavelengths of 100–150 km and >500 km.     

Stratigraphic and regional distribution of fractures in Barremian–Aptian carbonate rocks of Eastern Oman: outcrop data and their extrapolation to Interior Oman hydrocarbon reservoirs

The carbonates of the Barremian to Aptian Qishn Formation are outcrop equivalents to major hydrocarbon reservoirs in the Middle East and in Oman specifically. The rocks are exposed in the Haushi–Huqf area of eastern Oman where they are affected by pervasive jointing and localized folding and faulting. Information gathered in the Huqf outcrops can be used to formulate predictions on fracture patterns in adjacent reservoirs. Systematic joints are confined to few meters-thick intervals of widely differing lithologies, which can be correlated over hundreds of square kilometers. Over the entire area, systematic joints are typically more than tens of meters long, have spacings of 4–18 cm and homogeneous morphologies. These joints are interpreted to be of Late Aptian age. The dominant set of joints strikes consistently NW–SE and developed parallel to the causative maximum horizontal compression S_H. The direction of compression is at an high angle to the two major tectonic domains of the region, the subsiding Oman Interior basins and the elevated Haushi-Huqf High. NW–SE compression is proposed to have caused crustal/lithospheric buckling and thereby to have controlled Jurassic to Early Tertiary patterns of vertical movements. In such a scenario, the direction of compression is predicted to be constant over the entire domain. It is thus expected that Qishn carbonates in the subsiding Oman Interior basins also experienced NW–SE compression and developed systematic joints similar to those observed in the Huqf region. In the Neogene, with the establishment of the Zagros stress field, the maximum horizontal compression became roughly N–S, thereby possibly leading to the closure of pre-existing joint systems.     

Fracturing and fluid‐flow during post‐rift subsidence in carbonates of the Jandaíra Formation,Potiguar Basin,NE Brazil

Pervasive fracture networks are common in many reservoir‐scale carbonate bodies even in the absence of large deformation and exert a major impact on their mechanical and flow behaviour. The Upper Cretaceous Jandaíra Formation is a few hundred meters thick succession of shallow water carbonates deposited during the early post‐rift stage of the Potiguar rift (NE Brazil). The Jandaíra Formation in the present onshore domain experienced <1.5 km thermal subsidence and, following Tertiary exhumation, forms outcrops over an area of >1000 km². The carbonates have a gentle, <5?, dip to the NE and are affected by few regional, low displacement faults or folds. Despite their simple tectonic history, carbonates display ubiquitous open fractures, sub‐vertical veins, and sub‐vertical as well as sub‐horizontal stylolites. Combining structural analysis, drone imaging, isotope studies and mathematical modelling, we reconstruct the fracturing history of the Jandaíra Formation during and following subsidence and analyse the impact fractures had on coeval fluid flow. We find that Jandaíra carbonates, fully cemented after early diagenesis, experienced negligible deformation during the first few hundreds of meters of subsidence but were pervasively fractured when they reached depths >400–500 m. Deformation was accommodated by a dense network of sub‐vertical mode I and hybrid fractures associated with sub‐vertical stylolites developed in a stress field characterised by a sub‐horizontal σ₁ and sub‐vertical σ₂. The development of a network of hybrid fractures, rarely reported in the literature, activated the circulation of waters charged in the mountainous region, flowing along the porous Açu sandstone underlying the Jandaíra carbonates and rising to the surface through the fractured carbonates. With persisting subsidence, carbonates reached depths of 800–900 m entering a depth interval characterised by a sub‐vertical σ₁. At this stage, sub‐horizontal stylolites developed liberating calcite which sealed the sub‐vertical open fractures transforming them in veins and preventing further flow. During Tertiary exhumation, several of the pre‐existing veins and stylolites opened and became longer, and new fractures were created typically with the same directions of the older features. The simplicity of our model suggests that most rocks in passive margin settings might have followed a similar evolution and thus display similar structures.     

A Critical Analysis of the Wind Climatology of the Adriatic Sea from Different Sources

Summary  We consider the wind climatology of the Adriatic Sea derived from three different sources: an operational meteorological model, a simplified wind model, and traditional ship reports. The simplified wind model is found to provide reliable results in storm conditions, but it partly smoothes the fields when the meteorological pattern is not well defined and fairly uniform at the basin scale. The ship reports show a strong evident bias towards low values. This is partly interpreted as a tendency for ships to avoid rough weather and not to report in these types of conditions. A second reason is a bias present in the transfer from the Beaufort to the metric scale. The best results are provided by the operational meteorological model, after its results have been corrected for a bias in wind speed, associated with the resolution of the model with respect to the dimensions of the basin under study. Received October 23, 1997 Revised August 10, 1998     

The predictability of meteo-oceanographic events

We have explored the predictability of storms in a small enclosed basin with a complicated surrounding orography. We have considered two exceptional storms in the far past and three mild events happened in recent years. A posteriori forecasts have been done up to 6 days before the events. The results have been compared versus measured data and the related analysis. Good predictability (10–15% error in surface wind speed and wave height) have been found up to day 4, mildly larger (<30%) up to day 6 before the event. In no case was a storm missed. This suggests that the effective predictability in more open basins may extend to even larger ranges.     

Dynamic link between the level of ductile crustal flow and style of normal faulting of brittle crust

In a rheologically layered crust, compositional layers have an upper, elasto-plastic part and a lower, viscous one. When broken, the upper elastic part undergoes flexure, which is upward for the foot-wall and downward for the hanging wall. As a consequence of bending, stresses will develop locally that can overcome the strength of the plate and, therefore, impose the migration of active fault. In the lower, viscous part of each compositional layer, rocks can potentially flow. Numerical modelling of the behaviour of a crust made up of two compositional layers, during and following extension, shows that flow can take place not only in the lower crust but also, and more importantly, in the lower part of the upper crust. The ability of crustal rocks to flow influences the style and kinematics of rifted regions. When no flow occurs, subsidence will affect the extending areas, both hanging wall and foot-wall will subside with respect to an absolute reference frame such as sea level, and there will be a strict proportionality between extension and thinning. In addition, the downward movement of the fault blocks will decrease the local stresses created in the foot-wall and increase those of the hanging wall, thereby imposing a migration of fault towards the hanging wall. This is the behaviour of extensional settings developed on stabilised crust and which evolved in a passive margin. When flow does take place, middle crustal rocks will move towards the rifting zone causing isostatically driven upward movements that will be superimposed on movements associated with crustal and lithospheric thinning. Consequently, fault blocks will move upwards and the crust will show more extension than thinning. The upward movements will decrease the stresses developed in the hanging walls and increase those of the foot-wall. Faults will then migrate towards the foot-wall. Such a mode of deformation is expected in regions with thickened crust and has its most apparent expression in core complexes.     

The influence of a stratified rheology on the flexural response of the lithosphere to (un)loading by extensional faulting

We present a two-layered finite difference model for the flexural response of the lithosphere to extensional faulting. The model allows for three modes of flexure: (1) fully coupled, with the upper crust and mantle welded together by the lower crust; (2) fully decoupled, with the upper crust and mantle behaving as independent layers; and (3) partly decoupled, signifying that the response of the upper crust to small-wavelength loads is superimposed on the response of the entire lithosphere to long-wavelength loads. Which of these modes of flexure is to be expected depends on the rheology and especially the thermal state of the lithosphere. Coupled behaviour is related to a cold and strong lithosphere. The Baikal Rift Zone provides a typical example for this mode of flexure. A fully decoupled lithosphere is an exceptional case, related to anomalous high temperatures in the lower crust, and is observed in the Basin and Range province. The most common case is a partly decoupled lithosphere, with the degree of decoupling depending on the thickness and viscosity of the lower crust. This is inferred, for example, for the Bay of Biscay margin.     

Flexural response of the Venetian foreland to the Southalpine tectonics along the TRANSALP profile

The Venetian Basin was affected by flexure related to the Southalpine shortening phase during the Middle Miocene – Early Pliocene. This downbending is quantified here using a two‐dimensional flexural model. A recently improved data set on basin geometry based on the bottom of the Serravallian–Tortonian clastic wedge, on palaeobathymetry and gravity anomalies is used to constrain the components of flexure and to test the importance of the initial bathymetry in evaluating the contribution of surface loads to deflection. A good fit is obtained assuming a northward broken plate configuration of the downbent Adriatic plate with an effective elastic thickness of 20 km. Results highlight that, in the studied region, flexure related to the Eastern Southern Alps is totally due to surface loads (topographic load partly replacing initial bathymetry) and that no hidden loads are required. Furthermore, the palaeobathymetry contributes up to 50% to the total flexure in the studied region.