Évolution pression–température des amphibolites de la zone axiale au cours du métamorphisme hercynien des Pyrénées orientales

Thin levels of amphibolites from the Canigou, Albères and Cap de Creus massifs have been studied in order to investigate their pressure and temperature evolution during time. P and T values have been calculated using the amphibole–plagioclase–quartz thermo-barometer. Si, Al, Mg and Fe of zoned amphiboles have been analysed from core to rim by microprobe. By combining the results obtained from several (or different) crystals, P–T–t paths have been determined using the amphibole-plagioclase-quartz equilibriums. In the Canigou Massif, the amphibolites have recorded anti-clockwise P–T–t paths around a peak of metamorphism located at about 650?°C–6.1 kbar, whereas in the Albères Massif, the calculated P–T–t paths of amphibolites near the paragneisses are retrograde only, from 600?°C–5 kbar to 450?°C–2.5 kbar, but one cummingtonite-bearing amphibolite has also recorded an anti-clockwise evolution around 650?°C–4.5 kbar. The retrograde P–T–t paths recorded for amphibolites from the ‘Cap de Creus’ Massif are retrograde only, from 650?°C–6 kbar and 400?°C–1 kbar. To cite this article: C. Triboulet et al., C. R. Geoscience 337 (2005).     

Structuration profonde des dépôts de l'Albien–Maastrichtien en Tunisie centrale : nouvelle limite de l'archipel de Kasserine et implications géodynamiques

The Albian–Maastrichtian seismic horizon analysis in central Tunisia (Gafsa–Sidi Bouzid area) using the reflection seismic sections calibrated to the well data, shows buried structures with deposit distributions and sedimentation geometries varying from the depressive to uplifted zones. Pinch outs, unconformities and hiatuses recognized on the folded high structures are caused by reactivation of the bordering faults. The Turonian–Maastrichtian unconformities correspond to the palaeogeographic limits that outline the Kasserine Islets and correspond to the N120, N180 major wrench-salt-intruded corridors and associated N90, N60 strike-slip faults. Formation of the different structures and evolution of the basins and platforms were controlled by conjugate dextral and sinistral strike-slip movements. These structures allow new palaeogeographic limits of the Kasserine Islets to be identified. To cite this article: T. Zouaghi et al., C. R. Geoscience 337 (2005).     

L'unité ophiolitique de Pineto (Corse) : signification du détritisme continental dans sa couverture de flysch albo-cénomanien

The Jurassic N-MORB ophiolites of the Pineto Unit, which were unaffected by Alpine metamorphism, can be compared to the Apennine ophiolites. They are, however, distinguished by their cover rocks that include a silico-clastic flysch that we have dated as Albian–Cenomanian. Clastic deposits of the same type, but coarser grained, are known from the normal cover rocks of the Balagne Nappe E-MORB ophiolites, originally located on a thinned continental crust and/or near a continental margin. The Pineto Unit thus indicates that the detrital input of continental material was able to extend to a domain of clearly oceanic character in the Ligurian palaeo-ocean. To cite this article: M. Durand-Delga et al., C. R. Geoscience 337 (2005).     

Paléoclimat des Petites Antilles depuis 4000 ans BP : l'enregistrement de la lagune de Grand-Case à Saint-Martin

A 2.5-m-thick sequence of lake sediments at the Étang de Grand-Case, Saint Martin (French West Indies) is studied here. Significant hydrological budget fluctuations allows distinction of three main climatic periods: (1) an overall dry period (4200 BP–2300 BP), characterised by carbonated mud, gypsum and storm sand layers; (2) a wet phase (2300 BP–1150 BP) dominated by organic mud; and (3) a more complex phase (1150 BP to present), with detrital inputs due to human activities. Comparison with other regional high-resolution records shows that similar climate modifications typify the whole Mesoamerican and Caribbean area. The climatic phenomena that are implicated in the variations of both precipitation and hurricane frequency over the Lesser Antilles are due to the latitudinal displacement of the inter-tropical convergence zone. These data give new support to the hypothesis of the existence of a correlation between peopling phases and climate variations in the Caribbean as previously proposed by some archaeologists. To cite this article: P. Bertran et al., C. R. Geoscience 336 (2004).     

Privatization of rural industry and de facto urbanization from below in southern Jiangsu, China

At the heart of China’s economic transformation from plan to market is the institutional rearrangement of the ownership of property rights. Privatization of collectively owned township and village enterprises (TVEs) in recent years has significantly changed the rural economy and affected small town development in China. Located mainly in villages and small towns and producing 62% of the total industrial output in Jiangsu in 1994, TVEs played a crucial role in southern Jiangsu (Sunan)’s economic development and urbanization, giving rise to the well-known “Sunan model” of regional development during the 1980s. Since the mid-1990s, ownership transition (zhuanzhi) of the TVEs from collective to private in Sunan has altered the nature of the regional economy and the pattern of small town development. The privatization of TVEs in Sunan has been so pervasive that it can be seen as the “second industrial divide” in the region’s economy. This paper traces the evolution of ownership reform in Sunan’s rural industries leading to the current phase of privatization and analyzes the effects of privatization on de facto urbanization from below in Sunan. Given the importance of privatization, continuous scholarly monitoring of its relationship to regional economic development and urbanization from below is necessary.     

Middle Miocene peralkaline ignimbrites in the Hermosillo region (Sonora, Mexico): Geodynamic implications

Scattered ignimbritic mesas crop out in the Hermosillo region (Sonora, Mexico). These rocks that have been dated at 12.5 Ma (Middle Miocene) have the petrography and chemical characteristics of comendites. Such a flare-up of peralkaline acidic volcanism, after a long period of subduction-related arc volcanism, emphasises an important change in the source of volcanism. It corresponds to the latest stage of continental extension prior to the marine invasion and the development of spreading centres in the Gulf of California. To cite this article: J. Vidal Solano et al., C. R. Geoscience 337 (2005).     

The U-isotopes modeling in aquifers from Poços de Caldas plateau, Brazil

U-modeled groundwater residence times were estimated by changes in ²³⁴U/²³⁸U activity ratio (AR) and U content of groundwaters from Poços de Caldas city situated at the Poços de Caldas alkaline massif, Brazil. The estimated ages are more realistic than others generated by the use of hydraulic conductivity data and available information about the weathering rate of rocks in the plateau. The ²³⁴U/²³⁸U AR and reciprocal of the dissolved U-content data also defined a ternary plot that allowed the calculation of the relative volume of three source waters for the mixed water. Such U-isotopes modeling for mixing calculations indicated that the dominant phase (68%) in the mixture is thermal.     

Vegetation and climate change on the Bolivian Altiplano between 108,000 and 18,000 yr ago

A 90,000-yr record of environmental change before 18,000 cal yr B.P. has been constructed using pollen analyses from a sediment core obtained from Salar de Uyuni (3653 m above sea level) on the Bolivian Altiplano. The sequence consists of alternating mud and salt, which reflect shifts between wet and dry periods. Low abundances of aquatic species between 108,000 and 50,000 yr ago (such as Myriophyllum and Isoëtes) and marked fluctuations in Pediastrum suggest generally dry conditions dominated by saltpans. Between 50,000 yr ago and 36,000 cal yr B.P., lacustrine sediments become increasingly dominant. The transition to the formation of paleolake “Minchin” begins with marked rises in Isoëtes and Myriophyllum, suggesting a lake of moderate depth. Similarly, between 36,000 and 26,000 cal yr B.P., the transition to paleolake Tauca is also initiated by rises in Isoëtes and Myriophyllum; the sustained presence of Isoëtes indicates the development of flooded littoral communities associated with a lake maintained at a higher water level. Polylepis tarapacana-dominated communities were probably an important component of the Altiplano terrestrial vegetation during much of the Last Glacial Maximum (LGM) and previous wet phases.     

Geographical distribution of Tillandsia lomas in the Atacama Desert, northern Chile

We assessed the geographic distribution of Tillandsia lomas in northern Chile, from Arica (18°20′S) to the Loa river (21°25′S) and discussed the factors that might potentially underlie the observed patterns. We carried out extensive field survey complemented with aerial surveys and analysis of specimens deposited in herbaria. We detected over 30 Tillandsia stands most of which corresponded to the species Tillandsia landbeckii and can be grouped in 10 large systems. Other two species were also detected Tillandsia marconae and Tillandsia virescens, both of which show a restricted distribution in the area. Our results provide evidence on the wide distribution of Tillandsia lomas in northern Chile and its association with fog corridors.     

Using 2D inversion of magnetic resonance soundings to locate a water-filled karst conduit

A new methodology for magnetic resonance sounding (MRS) data acquisition and interpretation was developed for locating water-filled karst cavities. This methodology was used to investigate the Ouysse karst system in the Poumeyssens shaft in the Causse de Gramat (France). A new 2D numerical MRS response model was designed for improved accuracy over the previous 1D MRS approach. A special survey performed by cave divers confirmed the accuracy of the MRS results. Field results demonstrated that in favourable conditions (a low EM noise environment and a relatively shallow, large target) the MRS method, used with a coincident transmitter/receiver loop, can be an effective tool for locating a water-filled karst conduit. It was shown numerically that because an a priori orientation of the MRS profile with the karst conduit is used in the inversion scheme (perpendicular for instance), any error in this assumption introduces an additional error in locating the karst. However, the resulting error is within acceptable limits when the deviation is less than 30°. The MRS results were compared with an electrical resistivity tomography (ERT) survey. It was found that in Poumeyssens, ERT is not able to locate the water-filled karst. On the other hand, ERT provides additional information about heterogeneities in the limestone.