Unites structurales de la marge continentale sous-marine de Cannes a Menton d'apres la sismique refraction

Offshore seismic refraction profiles permitted us to characterize the extent of the geological formations which prolong the principal units of the Provence and Alpes Maritimes southwards and to establish their respective relationships. The concept of a “Var graben” is confirmed and defined. A magma chamber, probably in relation with the volcanic deposits of the Cap d'Ail, has been disclosed and coincides with the positive magnetic anomaly observed in this area.     

Influence of preglacial dispersion on geochemical exploration in county Offaly, Central Ireland

Weathering and erosion during the Tertiary period in Ireland gave rise to widespread secondary dispersion in the vicinity of exposed mineralization. Resulting patterns are still partly traceable under the glacial cover and constitute geochemical anomalies that are found to be more revealing than corresponding anomalies in the glacial till.A large near-surface lead and zinc anomaly in northeast County Offaly, Eire, was found to overlie an iron hydroxide-filled Tertiary(?) sinkhole. Tracing the dispersion patterns from this preglacial hydromorphic anomaly led to the discovery, to the north of the sinkhole and under the glacial drift, of two residual anomalies characterized by highly concentrated secondary zinc mineralization (up to 10% Zn). One of these follows a regional structural trend and may be of economic significance.Assessment of the depth of overburden was made using shallow refraction seismic equipment and sampling was carried out with two types of portable drills.     

North Pacific Radiolarian assemblages and their relationship to oceanographic parameters

A quantitative study of the distribution of Radiolaria in surface sediments of the North Pacific has demonstrated the feasibility of utilizing complete radiolarian thanatocoenoses as indicators of past oceanographic conditions.In samples from 36 core tops from the region 33° N-54° N, 148° E-140° W, 87–96% of all radiolarians encountered could be assigned to one of 120 taxa recognized. Q-mode factor analysis of distributional data for the 57 most abundant species yields four independent assemblages. Three of these reflect near-surface oceanographic conditions, and the fourth responds to bottom conditions. Regression-developed transfer functions describe the relationship between the assemblages and oceanographic parameters.     

Late Pleistocene history of the North Pacific: Evidence from a quantitative study of radiolaria in core V21-173

Objective quantitative estimates of paleo-oceanographic conditions in the North Pacific can be made by analyses of radiolarian assemblages. With appropriate computation, transfer functions developed in a study of surface sediments can be used to estimate oceanographic conditions in cores containing late Pleistocene radiolarian faunas. Analysis of core V21-173 indicates that conditions as warm as the Holocene were rare during the past 800,000 yr, and that the region experienced marked near-surface temperature drops correlative with Caribbean and continental records for the past 250,000 yr. A major world-wide warm event at about 400,000 yr is also indicated.     

Discovery of the Paleo-Tethys residual peridotites along the Anyemaqen–KunLun suture zone (North Tibet)

The spinel peridotite from the Anyemaqen suture contains ?5% residual clinopyroxene and is characterized by a high abundance of the magmaphile elements Fe, Al and Ti in the primary mineral phases. Our data demonstrate that this rock represents residual mantle material, which has been affected by less than 10% partial melting prior to its emplacement. Its textural features indicate that the rock has been plastically deformed in a non-coaxial regime under lithospheric physical conditions at a relatively cool thermal regime below solidus temperature. We suggest that the peridotites from the Anyemaqen suture represent mantle material, which was either emplaced during incipient rifting on the Palaeozoic passive margin of Asia, or uplifted at slow spreading ridge setting in Paleo-Tethyan Ocean. Further researches are needed to make a definite choice between these two alternatives. To cite this article: E.A. Konstantinovskaia et al., C. R. Geoscience 335 (2003).     

Assimilation of continental crust by komatiites in the Precambrian basement of the Carswell structure (Saskatchewan,Canada)

The isotope geochemistry (Sm-Nd, Pb-Pb and Rb-Sr) of mafic gneisses from the basement of the Carswell structure (Saskatchewan, Canada), rich both in Mg and incompatible elements (K, Rb, REE) has been investigated. A good Sm-Nd alignment gives a slope corresponding to an age of 3.7 Ga. However, comparison with major elements data strongly suggests that this alignment is a mixing line between Mg-rich, high CaO/Al₂O₃ magmas and the local felsic crust older than 2.9 Ga. The mafic magmas were probably of komatiitic affinity (MgO > 20 percent) but, nevertheless, were extracted from a source with nearly chondritic to slightly enriched light REE distribution. The age of the komatiite emplacement (1.9–2.9 Ga) is only loosely constrained by the oldest crustal residence age in the series and the subsequent metamorphic events. The granulite facies climax is dated at ca. 1.9 Ga by concordant whole rock Pb-Pb and Sm-Nd garnet-whole rock isochrons. The Rb-Sr systematics have been disturbed by later event(s) younger than 1.5–1.7 Ga, but do not permit a more precise assessment of the perturbation age.     

Sea-level estimates during the last deglaciation based on δO and accelerator mass spectrometry C ages measured in Globigerina bulloides

Coupled measurements of δ¹⁸O and accelerator mass spectrometry (AMS) ¹⁴C in a particular species of planktonic foraminifera may be used to calculate sea-level estimates for the last deglaciation. Of critical importance for this type of study is a knowledge of the seasonality of foraminiferal growth, which can be provided by δ¹⁸O measurements of modern shells (core tops, plankton tows). Isotopic (δ¹⁸O, AMS-¹⁴C dating) and faunal records (transfer function sea surface temperature) were obtained from two cores in the North Atlantic at about 37°N. The locations were chosen to obtain high sedimentation rate records removed from the major ice-melt discharge areas of the last deglaciation. Based upon Globigerina bulloides data, four δ¹⁸O-based sea-level estimates were calculated: −67 ± 7 m at 12,200 yr B.P. and −24 ± 8 m at about 8200 yr B.P. for core SU 81-18; −83 ± 10 m at 12,200 yr B.P. and −13 ± 11 m at about 8500 yr B.P. for core SU 81-14. Using a second working hypothesis concerning the seasonability of G. bulloides growth, it is suggested that the sea-level rose by about 40 m during the millennium which followed 14,500 yr B.P.     

Effects of rock anisotropy and heterogeneity on stress distributions at selected sites in North America

Provinces of regionally consistent horizontal stress orientations persist throughout the upper crust, and these may contain local areas where horizontal stress (σ_H) orientations seem to rotate relative to the regional stress orientations in a consistent manner. We suggest that the concept of aninclusion with anisotropy and heterogeneity of mechanical properties may be useful to explain some cases of σ_H rotations. σ_H rotations up to 50° and magnitude changes of 40% could reasonably be expected, based on simulations. Patterns correlate well with crustal observations; rotation of σ_H orientation reaches 25° to 58° in the different cases studied. We also model a strike-slip fault zone as a soft inclusion, showing reorientations of σ_Hmax which agree with observed rotations near the Murre Fault in the Jeanne d'Arc Basin, Canada. Results imply that understanding the behaviour of soft and stiff inclusions, or expanding and contracting thermal inclusions for that matter, can help explain a number of geophysical and geological phenomena related to stress patterns.