Computation of analytical partial derivatives of phase and group velocities for Rayleigh waves with respect to structural parameters

Summary The analytical expressions used to compute the partial derivatives of phase and group velocity of Rayleigh waves with respect to the P- and S-wave velocity and the density are derived and the related computer code is developed. The results of the analytical computations were satisfactorily tested against numerically determined values. Several examples of partial derivatives for a given structural model are presented.     

The Dorn gold deposit in northern Victoria Land,Antarctica: Structure,hydrothermal alteration,and implications for the Gondwana Pacific margin

The Dorn gold deposit in northern Victoria Land, Antarctica is a fault related gold-only deposit and it represents the first described occurrence of gold mineralization in Paleozoic terranes of the Antarctic continent. The deposit is hosted in lower greenschist facies Middle Cambrian metavolcanic and metasedimentary rocks of the Bowers terrane. Gold-bearing veins are located in a brittle–ductile reverse high-strain zone, which was produced by transpressional deformation that overprints the structures of the Cambrian–Ordovician Ross Orogeny. The vein system is surrounded by a hydrothermal alteration zone that is as much as 300-m-wide, where the host rocks are partially to completely transformed into Fe–Mg carbonate-rich rocks with different degrees of replacement of the original mineralogy and texture.The type of host rock, the temperature estimates for mineralization from 290–320 °C, the quartz dominant vein system with sulfides and Fe-rich carbonates, and the controlling structures linked to a convergent margin tectonic setting together suggest that this mineralized vein system can be classified as an orogenic gold deposit. Close analogies are found with deposits of the Stawell zone in western Victoria, which is consistent with the correlation between units and hydrothermal events in northern Victoria Land and southeastern Australia.     

GCM sensitivity experiments with locally modified land surface properties over tropical South America

Ensembles of 1-year-long experiments with a relatively high-resolution ECMWF model were conducted in order to investigate the impact of modified land surface properties on local, regional and large-scale atmospheric circulations. The modifications consisted of changes to land cover and increased albedo over the northern part of South America. In many respects the experimental design resembles the setting of classical deforestation experiments. The local model response to imposed modifications, which includes a reduction in precipitation as well as in evaporation and an increase in surface temperature, was found to be stronger in dry (July–September, JAS) than in wet (January–March, JFM) season, and in the ensemble with higher albedo value. Local drying is discussed in terms of locally generated overturning that resembles a direct thermal circulation. The effects of this circulation seem to be dominant over the reduction in large-scale moisture supply from the adjacent ocean. On large scales, changes to the Pacific branch of the Walker circulation lead, through modified divergent flow, to a tropics-wide impact on precipitation. In addition to South America, the largest changes are seen in the south Pacific convergence zone in JFM, while the impact on the Atlantic inter-tropical convergence zone is stronger in JAS. In the extratropics, there is little change in precipitation. In the upper troposphere, a distinctive teleconnection wave-pattern could be seen in the Pacific/North American region during JFM. A notable feature in the upper-air model response in JAS is a wave train extending from South America, over the northern Atlantic into Europe. With regard to the interaction between the land surface response and model systematic errors, our results suggest that the erroneous shift of the downward branch of the Pacific/South American Walker circulation is likely to be a cause, rather than a consequence, of the rainfall deficit over South America in the model climatology.     

Deformation and metamorphism in the Fenes Nappe (southern Apuseni Mountains,Romania)Déformation et métamorphisme dans la nappe de Fenes (monts Apuseni méridionaux,Roumanie)

The Fenes Nappe belongs to the stack of tectonic units cropping out in the southern Apuseni Mts (Romania). It is characterised by a structural history consisting of two folding phases that developed during the time spanning from Early Aptian to Late Maastrichtian. The D1 phase produced west-northwest-verging, isoclinal to very tight folds, associated to a slaty cleavage. The main metamorphic imprint of the Fenes Nappe is linked to this deformation phase; illite and chlorite ‘crystallinity’ values indicate metamorphic conditions of the late diagenesis, close to the diagenetic zone/anchizone boundary. The subsequent D2 phase produced north-northwest-verging, parallel folds, not associated with synkinematic recrystallisation. These phases are interpreted as developed during a structural path, which includes burial at a depth of 8–10 km, followed by exhumation at shallower structural levels. To cite this article: A. Ellero et al., C. R. Geoscience 334 (2002) 347–354.     

Pesticide contamination of the coastline of Martinique

In January and February 2002, the presence of certain agricultural pesticides throughout the coastline of the Caribbean island of Martinique was investigated. The tropical climate of the French West Indies is suitable for banana production, which requires intensive use of pesticides. An inventory of all pesticides used on the island (compounds and tonnage) was compiled. Surveys and analyses revealed the presence of pesticides in the plumes of seven rivers. The organochlorine chlordecone and metabolites of aldicarb were detected at nearly all of the monitored sites, even though the use of chlordecone has been prohibited since 1993. Two triazines (ametryn and simazine) were also identified. The concentrations of carbamates and triazines detected in the water and sediment samples from Martinique are comparable to those reported for mainland France. Chlordecone concentrations in the sediment and particulate matter samples were, however, particularly high in the samples from Martinique. Toxicological implications are discussed. Of particular concern are the high levels of chlordecone (which is bioaccumulating and carcinogenic) and further monitoring of this compound is recommended, especially in fish and other sea-food products.     

Recycling of palaeo-Pacific subducted oceanic crust related to a Fe–Cu–Au mineralization in the Xu-Huai region of North Anhui-Jiangsu,East China: Geochronological and geochemical constraints

ABSTRACT

In this study, Early Cretaceous skarn deposits and genesis of their host diorite/monzodiorite porphyry in the Xuzhou-Huaibei (Xu-Huai) region, northern Anhui-Jiangsu have been discussed by detailed geochemical work. In-situ zircon U–Pb dating of the diorites related to Fe–Cu–Au deposits shows that they were formed between 131.4 ± 1.5 Ma and 130.8 ± 1.8 Ma. Geochemical data indicate a depletion of high field strength elements (HFSE) in the diorite porphyry with similarity to that of arc-related igneous rocks. The diorite porphyry was probably derived from typical arc magmas related to continental margin subduction characterized by light rare earth elements (LREEs) enrichment and HFSE depletion. REEs compositions of apatite in the diorite porphyry indicate that the dioritic magma was produced from the metasomatized subcontinental mantle by slab-derived fluids. The magma was proven to be a high oxygen fugacity; thus, it was particularly conducive to the precipitation of Fe, Cu, Au and other ore-forming elements. The δ³⁴S values of pyrite and chalcopyrite of Fe–Cu–Au ores range from ?0.2‰ to 2.8‰, indicating that the sulphur in the ore was probably derived from deep-seated magmas. Integrated with geochronological and geochemical analyses, we suggest that the Early Cretaceous igneous suites associated with Fe–Cu–Au deposits in the Xu-Huai region are related to recycling subduction of Pacific oceanic crust.     

Possible correlation between a mantle plume and the evolution of Paleo-Tethys Jinshajiang Ocean: Evidence from a volcanic rifted margin in the Xiaru-Tuoding area, Yunnan, SW China

Several Paleozoic sutures in Southwestern China provide a record of the history of the Paleo-Tethys Ocean, whose birth and final closure are associated with the breakup and assembly of Gondwanaland. Recent studies indicate that there are widespread OIB-type mafic volcanic rocks within these suture zones and intervening terranes. This paper examines the geology and geochemistry of volcanic rocks in the Xiaruo-Tuoding area, a remnant passive margin succession of the Jinshajiang Paleo-Tethyan Ocean. The sedimentary and volcanic stratigraphy of this area is interpreted as a seaward dipping margin with a few continentward dipping normal faults. The available geochemistry of these volcanic rocks suggest that they are OIB-like basalts, characterised by SiO₂ = 42.78–50.46 wt.%, high TiO₂ contents (TiO₂ = 2.2–3.55 wt.%), moderate MgO = 4.15–6.49 wt.%, Mg# = 0.37–0.50, high Ti/Y ratios (mostly > 450), large ion lithosphere elements enrichment, high strength field elements and rare earth elements, with La/Nb = 1.04–1.39, Ce/Yb = 18.38–30, Sm/Yb = 2.16–3.52, (⁸⁷Sr/⁸⁶Sr)i = 0.705350–0.707867, and Nd(t) = − 1.43–1.90. These geochemical and isotopic signatures are generally similar to those of the Emeishan flood basalts, which together with stratigraphic constraints, demonstrate that these volcanics were formed in a volcanic rifted margin, probably associated with a mantle plume. A new model is proposed to interpret the evolution of the Jinshajiang Paleo-Tethyan Ocean and its possible relationship to the Emeishan mantle plume. In this model, we argue that the opening of the Jinshajiang Paleo-Tethyan Ocean in the Carboniferous was caused by a mantle plume. The mantle plume was active to the east along the western margin of the Yangtze Craton between 300 and 260 Ma, from which the voluminous Emeishan flood basalts were erupted at 260 Ma. The closure of the Jinshajiang Ocean occurred since the Middle Permian. Continuous westward subduction generated the Jiangda-Weixi magmatic arc to the west of the Jinshajiang suture. This subduction also partly destroyed and/or tectonically sliced the volcanic rifted margin. Some seaward dipping volcanic-sedimentary sequences on the east flank of the Jinshajiang Ocean were preserved, but are strongly deformed.     

Rock-magnetic and paleomagnetic results from the Tepic-Zacoalco rift region (western Mexico)

A rock-magnetic and paleomagnetic investigation was carried out on eleven Pleistocene and Pliocene ⁴⁰Ar/³⁹Ar dated lava flows from the Tepic-Zacoalco rift region in the western sector of the Trans-Mexican Volcanic Belt (TMVB) with the aim of obtaining new paleomagnetic data from the study region and information about the Earth’s magnetic field recorded in these rocks. Rock-magnetic experiments including measurement of thermomagnetic curves, hysteresis parameters and isothermal remanence acquisition curves were carried out to find out the carriers of remanent magnetisation and to determine their domain structure. Although some samples were characterised by the presence of a single ferromagnetic phase (magnetite), in most cases more phases were observed. Analysis of hysteresis parameters showed a mixture of single domain and multidomain particles, the fraction of the latter varying between 40% and 80%. Paleomagnetic results were obtained in all sites, although in 7 sites characteristic remanence directions and remagnetisation circles had to be combined in order to calculate site means. The six Pliocene sites not showing intermediate polarity yielded a paleomagnetic pole (latitude ? = 81.1°, longitude λ = 94.3°) which roughly agrees with the expected one. Paleomagnetic directions do not indicate significant vertical-axis block rotations in the western TMVB area. Reversed polarities observed can be correlated to the Gilbert chron, normal polarities to the Gauss chron or the Brunhes chron and intermediate polarities to the Cochiti-Gilbert or the Gilbert-Gauss transition. The reversed or intermediate polarity magnetisation recorded in one of the sites (542 ± 24 ka) corresponds either to the West Eifel 4 or the West Eifel 5 excursion, while the reversed polarity observed in the other site (220 ± 36 ka) very likely provides new evidence for the Pringle Falls excursion or the event recorded in the Mamaku ignimbrite.