A note on the Goodman Jack

SummaryA Note on the Goodman Jack Reconnaissance experiments, performed to evaluate the practical utility of the hard-rock variety of the Goodman Jack, reveal that the Hustrulid-T^* correction adequately reconciles the discrepancy between the measured and true deformation modulus of the rock mass in the range of 30 to 50 gigapascals.With 3 Figures     

Where were the Pan-African mountains? No evidence of 500 m.y. detrital zircons

Evidence for or against the collision hypothesis for the origin of intracontinental mobile belts, such as the Pan-African, is difficult to devise in view of the complexities of collision sutures. We propose that the simple criterion of elevation, with subsequent deep erosion of crystalline rocks which shed isotopically dateable zircons into adjacent sediments, such as in the case of the Appalachians, may provide such evidence. Zircons from the Phanerozoic sedimentary deposits in northern Africa, collected in Tunisia, Morocco, Sicily and Egypt, have been found to contain radiogenic lead which in all cases lies along primary chords of about 1750 m.y. in a Concordia plot. Evidence in the data points to the absence of any significant contribution of Pan African zircons (about 500 m.y.) in any of the samples, which cover a range in time of deposition from Ordovician to the present. From this evidence we conclude that the extensive Pan-African mobile belts which should have developed great chains of mountains over a large proportion of Africa if they resulted from subduction or collision, were not of that origin.     

Dissolved organic matter in pore water of carbonate sediments from Bermuda

Pore water samples from seven nearshore areas in Bermuda were obtained under in situ conditions and analyzed for dissolved organic carbon, dissolved carbohydrates, dissolved free amino acids and dissolved humic substances. The concentration of dissolved organic carbon is higher than in the overlying nearshore waters indicating significant diagenetic remobilization of carbon in these recently deposited carbonate sediments. Dissolved carbohydrates decrease with depth due to microbial utilization.     

La mesure de l'anisotropie de susceptibilité magnétique dans les flyschs: Application à la recherche des directions des paléocourants

Studies made on the anisotropy of magnetic susceptibility of artificial sediments have shown that there are correlations between the orientation of anisotropy axes and of transport directions. Thus, the analysis of parameters of anisotropy of magnetic susceptibility can constitute a means to investigate transport directions in flysch. To test this method in a natural model, the authors have made some measurements on flysch beds belonging to the “grés d'Annot complex”. The sediments have been sampled in the Contes and Peira-Cava synclines where the directions of paleocurrents have been determinated by classical methods. Results obtained both in Contes and in Peira-Cava on fine-grained detrital beds show a good agreement between directions of paleo-transport deduced from orientation of anisotropy axes and those established by classical methods on the levels showing “sole-marks”. However, in some beds of the Peira-Cava syncline, which are characterized by coarser grain size and “sole-marks” of good quality, the scattering of the directions of the anisotropy axes is very great and in some cases the measurements are not reproducible in the same sample.It appears, therefore, that the measurement of the anisotropy of magnetic susceptibility can constitute a good approach to analysing emplacement modes of fine-grained flysch deposits in which the use of classical methods is often difficult. This method of paleocurrent analysis complements the other techniques used in this field of research.     

Geochemical morphology of the North Mid-Atlantic Ridge, 10°–24°N: Trace element-isotope complementarity

The new data presented here from a 10–24°N segment of the North Mid-Atlantic Ridge show that this segment is the most depleted of the 10–70°N ridge section. They also show the existence of: (1) a geochemical gradient from the 14°N anomaly to 17°10′N; (2) a very depleted mantle source (the lowest Sr isotopic ratios found so far in the North Atlantic); and (3) a geochemical limit located at about 17°10′N without any obvious relation with any structural feature. The 15°20′N fracture zone does not show any relationship with respect to this gradient. The basalts located north of 17°10′N have very homogeneous features, which allow their characteristics to be averaged (i.e., ⁸⁷Sr/⁸⁶Sr= 0.70238 ± 0.00004, (Nb/Zr)_N = 0.28 ± 0.1) and they are defined as normal mid-ocean ridge basalts. The basaltic glasses located south of 17°10′N present a wide spectrum of isotopic compositions and extended rare earth element patterns (from depleted to enriched). Despite this, they have a constant K/Nb of 233 ± 9 (1s_M, n = 18) whereas this ratio is 344 ± 29 north of 17°10′N. These observations illustrate the strong coherence of behaviour between K and Nb (Ta) during the petrogenic processes involved in the generation of these mid-ocean ridge basalts and also their fractionation during previous mantle processes. Possible interpretations of mixing processes are discussed and sources at the ridge segment scale are favoured. However, when looking in detail, local heterogeneities are still common and can even be traced back off-axis to 115 my.

Placed in the context of the North Atlantic Ridge from 10° to 70°N, the Sr isotopic ratios reveal the Azores superstructure (23–50°N), whereas the trace element ratios (La/Sm-Nb/Zr) trace the second-order structures (33–40°N, 42–48°N) superimposed on the superstructure. This study illustrates the complementarity of information given by certain well chosen trace element ratios on the one hand and by isotopic ratios on the other. Since there is evidence of decoupling between isotopic ratios and/or trace element ratios, it introduces the notion of complementary “chemical memory” as recorded by a given type of trace element ratio or a given type of isotopic ratio     

Utilisation d'un microdébitmètre à haute résolution pour les mesures de turbulence par avion

Résumé Nous avons décrit dansBardeau et al. (1987), la réalisation, le fonctionnement et les tests d'un microdébitmètre basé sur l'anémométrie à fil chaud. Il a été particulièrement montré que cet appareil était bien adapté aux mesures de pression et pression différentielle avec une grande sensibilité et une très faible constante de temps. Nous présentons ici les applications qui ont été faites de ce capteur aux mesures à haute résolution à bord d'avions instrumentés pour la recherche atmosphérique. L'avion utilisé était spécialement équipé pour les mesures de turbulence et permettait donc l'acquisition d'un grand nombre de paramètres indispensables à l'analyse des données de ce capteur. Dans les applications aéronautiques qui ont été faites, le capteur a surtout été utilisé en tant que variomètre. Les comparaisons avec les mesures standards d'altitude par pression et couplages pression-données inertielles montrent que cet instrument pourrait apporter une contribution intéressante dans le domaine de la mesure à haute fréquence des fluctuations de pression.     

Deccan flood basalts at the Cretaceous/Tertiary boundary?

Joint consideration of new paleomagnetic, paleontological and geochronological data from the Deccan continental flood basalts in India and critical discussion of earlier results lead us to suggest that volcanic activity may have lasted less than 1 Ma, thus possibly ranking as one of the largest volcanic catastrophes in the last 200 Ma. Available data are best satisfied if volcanism spanned the Cretaceous/Tertiary boundary, followed shortly afterwards by rifting of the Arabian Sea. These results point out the need for further work which may help in choosing between “external” and “internal” models of the Cretaceous/Tertiary boundary events.