Circumgranular bassanite in a gypsum crust from eastern Algeria – a potential palaeosurface indicator

Detrital sand grains are surrounded by thin bassanite coatings in the upper part of a coarse-crystalline gypsum crust from the Algerian Sahara. The bassanite developed by topotactic replacement of the surrounding gypsum in the absence of a liquid phase. Heating experiments using a gypsum crystal with sand inclusions produced similar patterns and textures. In one experiment, bassanite developed around quartz and carbonate grains but not along the sides of the heated gypsum crystal. This is the result of differences in heat capacity between gypsum, quartz and calcite. Bassanite formation in the crust from Algeria was not controlled by differences in thermal properties. Instead, the bassanite apparently formed under conditions of thermal equilibrium. The occurrence of bassanite as circumgranular coatings in the crust is interpreted as being related to the availability of submicroscopic space along the contact between the gypsum cement and the enclosed sand grains. The presence of coatings of this type, or derived relict features, is a potential criterion for the recognition of palaeosurfaces.     

Influence of Mediterranean sea-level changes on the Dacic Basin (Eastern Paratethys) during the late Neogene: the Mediterranean Lago Mare facies deciphered

A recently published scenario viewing the Messinian salinity crisis as two evaporitic steps rather than one has led to a search for new indices of the crisis in the Eastern Paratethys. Fluvial processes characterized the southwestern Dacic Basin (Southern Romania, i.e. the Carpathian foredeep) whereas brackish sediments were continuously deposited in its northern part. This is consistent with previously evidenced responses of the Black Sea to the Messinian salinity crisis. High sea‐level exchanges between the Mediterranean Sea and Eastern Paratethys are considered to have occurred just before and just after desiccation of the Mediterranean. This accounts for two successive Mediterranean nannoplankton‐dinocyst influxes into the Eastern Paratethys that, respectively, belong to zones NN 11 and NN 12. Meanwhile, two separate events that gave rise to Lago Mare facies (with Paratethyan Congeria, ostracods and/or dinoflagellate cysts) arose in the Mediterranean Basin in response to these high sea‐level exchanges and located 5.52 and 5.33 Ma (isotopic stages TG 11 and TG 5, respectively), i.e. just before and just after the almost complete desiccation of the Mediterranean). These Lago Mare facies formed independently of lakes with ostracods of the Cyprideis group that developed in the central basins during the final stages of desiccation. The gateway faciliting these water exchanges is not completely identified. A proto‐Bosphorus strait seems unlikely. A plausible alternative route extends from the northern part of the Thessaloniki region up to the Dacic Basin and through Macedonia and the Sofia Basin. The expression ‘Lago Mare’ is chronostratigraphically ambiguous and should be discontinued for this purpose, although it might remain useful as a palaeoenvironmental term.     

Shear Wave Anisotropy of the Upper Mantle Beneath the Region from Tingri of Tibet to Golmud of Qinghai

Measurements of shear wave splitting at 43 three-component seismic stationsshow very big difference in anisotropy on both sides of the Indus-Yarlung Zangbo suture(ITS), but little difference on both sides of the older Bangong-Nujiang suture (BNS) and theJinsha River suture (JS) to its north. Obvious discrepancy exists between the anisotropy direc-tion and the superficial tectonic trends, which is not explicable directly by the coherent uppermantle deformation usually supposed to occur in consistency with the trend of a collisional belt.On the other hand, strong spatial relationships are observed from the anisotropy results, such asthe orthogonal directions of anisotropy on both sides of ITS and the good correlation betweenthe region of larger magnitude of anisotropy and the zone of inefficient Sn propagation ofQiangtang as well as the systematic rotation of the directions of anisotropy, which should testifysome much more complicated aspects of the continental convergence mechanism. To the best ofour data, we tend to suppose that the Qinghai-Tibet plateau might result from a mechanismcomplicated by the coexistence of Argand's underthrusting and Dewey's diffuse deformation.     

'Race' as a scientific and organizational construct: a critique

'Race' for many years has been a major construct of science and society. While its importance as such has not historically been particularly pronounced on a global scale, the emergence of its most forceful architects, the Anglophone countries, to pre-eminence since World War II has significantly extended its geographical range and added to its significance as an idea within commercialized culture as well as within social organization.In the present paper, 'race' is critically examined from the following angles: 1) its role in the behavioral and medical sciences; b) its historical origins and manifestations within the Anglophone countries, particularly the United States; and c) its emergence as a 'liberal' concept and operating principle since World War II. Questions of why and how 'race' arose and its continued use in science, society and culture drive both the trajectory and depth of this research. 'Race' is found to be a modern construct which arose as a consequence of colonialism and slavery, and was substantially constructed in its present form and substance by England and its off-spring societies, particularly the United States. 'Race' was not used as a term expressing a social idea until modern times, and had no basis in the primordial civilizations which greatly influenced modern Western societies (e.g., ancient Greece and Rome). Efforts undertaken by liberals – particularly in the United States – to 'humanize' the concept of 'race' since the 1960's have been largely unsuccessful. 'race' is viewed as inherently hierarchical, a fact which is evident from its historical and present role in science and society.     

The structure of the guinean continental margin: Implications for the connection between the central and the south atlantic oceans

The results of a recent geological-geophysical survey, conducted off Guinea, are combined with previous data to establish a preliminary stratigraphy and provide a structural sketch of this portion of the West African continental margin. Three sectors are distinguished:

A northwestern portion of the margin comprises a wide and deeply submerged plateau — the Guinean Marginal Plateau underlain by a thick sedimentary sequence and facing westward toward the Gambia Abyssal Plain. Scismic stratigraphy and structures show clear analogies to the Jurassic margins of the central Atlantic. Including the presence of a Cretaceous paleoslope covered by Cenozoic deposits.

A southern area of the margin comprises a series of aligned (W-E trending), acoustic basement features extending along the slope and bounding the Guinean Plateau to the south. These features, basement ridges and volcanic piles are related to a fracture zone system also documented by magnetic anomalies and gravity data. The bordering deep Sierra Leone abyssal plain, also dissected by E-W-trending oceanic fracture zones, contains a sedimentary cover apparently not older than middle Cretaceous.

Between both sectors and between two NW-SE trending scarps lies an intermediate area. The seismic profiles show that here, the margin is dissected by faults creating a series of asymmetric horst and graben features progressively narrowing towards the S-E and covered by untectonized (but partly eroded) Upper Cretaceous to Cenozoic sediments.

The overall structure of the Guinean Margin is interpreted as the result of two major events. During a first phase the margin was created at the southern extremity of the central Jurassic Atlantic and developed like other comparable margins. During a s econd phase (beginning in Early Cretaceous times) the margin was progressively submitted to the opening of the equatorial South Atlantic. This process gave rise to the margin of the southern Guinean plateau (locally injected by volcanics) and generated the tectonic features of the intermediate zone. This protion may thus represent a part of the rifted Jurassic margin discordantly dissected by the oblique opening of the south Atlantic in the area. The oceanic crust of the central and south Atlantic were definitively connected only during Late Albian times as indicated by the end of the tectonic activity and the early Upper Cretaceous unconformity.     

Dynamic-stiffness matrix of soil by the boundary-element method: Conceptual aspects

Starting from a weighted-residual formulation, the various boundary-element methods, i.e. the weighted-residual technique, the indirect boundary-element method and the direct boundary-element method, are systematically developed for the calculation of the dynamic-stiffness matrix of an embedded foundation. In all three methods, loads whose analytical response in the unbounded domain can be determined are introduced acting on the continuous soil towards the region to be excavated. In the weighted-residual technique and in the indirect boundary-element method, a weighting function is used; in the latter case, it is selected as the Green's function for the surface traction. In the direct boundary-element method, the surface traction along the structure-soil interface is interpolated. The same type of boundary matrices which have a clear physical interpretation are identified in the three formulations, each of which is illustrated with a simple static example. The indirect boundary-element method leads to the most accurate results. The guaranteed symmetry and the fact that the displacement arising from the applied loads can easily be calculated and compared to the prescribed displacement makes the indirect boundary-element method especially attractive for calculating the dynamic-stiffness matrix of the soil. Instead of calculating the dynamic-stiffness matrix of the embedded foundation with the boundary-element method, it can be determined as the difference of those of the regular free field and of the excavated part. The calculation of the former does not require the Green's function for the surface traction. The dynamic stiffness of the excavated part can be calculated by the finite-element method.     

Dynamic-stiffness matrix of soil by the boundary-element method: Embedded foundation

Green's influence functions are derived for a linearly distributed load acting on part of a layered elastic halfplane on a line which is inclined to the horizontal. Using these Green's functions as fundamental solutions in the boundary-element method, the dynamic-stiffness matrices of the unbounded soil with excavation, of the excavated part and of the free field are calculated. The indirect boundary-element method using distributed loads and no offset leads to more accurate results than the weighted-residual technique and the direct boundary-element method. At the natural frequencies of the undamped excavated part built-in along the structure-soil interface, the spring coefficients associated with the dynamic-stiffness matrices of the excavated part and of the free field will become infinite. If the dynamic-stiffness matrix of the soil with excavation is calculated as the difference of that of the free field and that of the excavated part, the difference of two large numbers will arise in the vicinity of these frequencies. A consistent discretization must then be used. In particular, the dynamic-stiffness matrix of the embedded part cannot be determined by the finite-element method in this case. A parametric study is performed for the dynamic-stiffness matrix of the free field for a rectangular foundation embedded in a halfplane and in a layer built-in at its base; the aspect ratio and the damping of the soil are varied.     

The relative deformation gradient and its application to cylindrical folds with deformed lineations

The relative deformation gradient is defined and used to compare deformations between any two points of a body. This concept is useful in the geometrical analysis of deformation in natural folds, where only deformed planes and straight lines can be observed. The case of cylindrical folds with deformed lineations is studied.     

The characteristics and dynamics of tar pollution on the beaches of Trinidad and Tobago

Trinidad and Tobago beaches were periodically sampled for tar between July 1980 and September 1981. This project was carried out by the Institute of Marine Affairs in cooperation with the United Nations. The Caribbean coasts of both islands were almost pristine, whereas pollution on the Atlantic coast beaches was comparable to that of other coasts along major tanker routes. The source of high tar concentrations on the Atlantic coasts may be attributed to the residues from tanker bilge cleanings, which are carried along the South Equatorial Current, eventually to strand on the islands' beaches. In the dry season (January to April) north-westerly currents and north-easterly winds prevail and more tar strands on Trinidad than on Tobago. In the wet season (June to November), however, when currents are more northerly and south-easterly wind prevail, the reverse situation holds and more tar strands on Tobago.     

Potential Flank-Collapse of Soufrière Volcano, Guadeloupe, Lesser Antilles? Numerical Simulation and Hazards

The past history of recurrent flank collapses of la Soufrière volcano of Guadeloupe, its structure, its well-developed hydrothermal system and the current activity constitute factors that could promote a future flank collapse, particularly in the case of a significant increase of activity, with or without shallow magmatic input. To address the hazards associated with such a collapse, we model the emplacement of the debris avalanche generated by a flank-collapse event in 1,250 BC (3,100 years B.P.). We use a finite-difference grain-flow model solving mass and momentum conservation equations that are depth-averaged over the slide thickness, and a Coulomb-type friction law with a variable basal (minimum) friction angle. Using the parameter values determined from this simulation, we then simulate the debris avalanche which could be generated by a potential collapse of the present lava dome. We then discuss the region which could be affected by such a future collapse, and additional associated hazards of concern.