Les filons sédimentaires jurassiques de Longobucco-Caloveto (Calabre,Italie); application à l’étude des paléostructures d’une marge téthysienne

Résumé

La région de Longobucco-Caloveto, dans le Nord du massif de la Sila, en Calabre, montre de très nombreux filons sédimentaires jurassiques qui traversent des schistes cambro-ordovicien ou des granites hercyniens. Ces filons se répartissent en deux groupes. Les plus anciens, d’âge carixien, sont associés à des failles normales qui ont permis la formation d’un demi-graben rempli par des sédiments du Domérien. Les plus récents, d’âge compris entre le Lias supérieur et le Malm, marquent la fracturation de hauts-fonds qui se sont effondrés au Dogger. Ces filons enregistrent ainsi la formation du segment de la marge de la Téthys auquel correspondent les séries de Longobucco-Caloveto. L’étude des relations entre les filons sédimentaires et la fracturation permet de montrer, dans une région pourtant fortement tectonisée au Tertiaire, que différentes failles sont d’âge jurassique : certaines d’entre-elles sont recoupées par des filons et donc antérieures à ceux-ci; d’autres systèmes de filons sont parallèles à des systèmes de failles et donc postérieurs à ces derniers ou contemporains. Us filons sédimentaires apparaissent ainsi comme un moyen intéressant, bien qu’encore peu utilisé, de caractén-sation des dispositifs paléotectoniques.     

Tectonic evolution of the Montagne Noire (french Massif Central): a model of extensional gneiss dome

Abstract

The tertonic interpretation of the Montagne Noire Gneiss Dome (southern French II Massif (Central) has been controversial for a long time. Several models have been proposed : diapirie uplift, wreneching and diapirism, compressive anticline, and metamorphie core complex. Evidence for extensional tectonics in the French Varisean Belt favours the latter interpretation. Strain and metamorphism patterns in the eastern part of the Montagne Noire result from two successive extensional deformations during Late Carboniferous to Permian times. The occurrence of a major detachment zone along the northern edge of the Montagne Noire Gneiss Dome as well the presence of sedimentary) basias to the north point to the asymmetry) of the Stephanian-Permian extensional system. We propose a new model of gneiss dome involving isostatie uprising and consecutive tectonic denudation of the duetile lower crust. This process results in asymmetrical extensional systems characterized by roll-under folding of the footwall and development of basins in the hangingwall as in the Montagne Noire. The model is finally discussed in comparison to previous interpretations.     

Stretching lineation and transport direction in the Ibero-Armorican arc during the siluro-devonian collision

Abstract

Large structures, lineations, foliations and sense of shear criteria are examined on the scale of the whole Ibero-Armorican Arc. Four sections (Galicia, Brittany-Vendée, Limousin and Eastern Massif Central) exemplify the major thrust sheets observed around the Arc. Stretching lineations are contemporaneous with the siluro-devonian metamorphism and are either transverse, oblique or parallel to the collision zone. A kinematic analysis shows that these lineations have resulted from a dominanüy transverse shear deformation which was followed by, or combined with, a longitudinal shear direction. On the scale of the entire Arc, this variation in the shear direction is interpreted as resulting from an early head on thrusting relative movement evolving to large scale movements parallel to the plate boundaries. Experiments with sand-silicone models support a model which generates the Arc by interaction between a transform sinistral direction, and a converning zone at a high angle to the transform direction.     

Seasonal variability of cohesive sediment aggregation in the Bach Dang–Cam Estuary, Haiphong (Vietnam)

In the Bach Dang–Cam Estuary, northern Vietnam, mechanisms governing cohesive sediment aggregation were investigated in situ in 2008–2009. As part of the Red River delta, this estuary exhibits a marked contrast in hydrological conditions between the monsoon and dry seasons. The impact on flocculation processes was assessed by means of surveys of water discharge, suspended particulate matter concentration and floc size distributions (FSDs) conducted during a tidal cycle at three selected sites along the estuary. A method was developed for calculating the relative volume concentration for the modes of various size classes from FSDs provided by the LISST 100X (Sequoia Scientific Inc.). It was found that all FSDs comprised four modes identified as particles/flocculi, fine and coarse microflocs, and macroflocs. Under the influence of the instantaneous turbulent kinetic energy, their proportions varied but without significant modification of their median diameters. In particular, when the turbulence level corresponded to a Kolmogorov microscale of less than ∼235 μm, a major breakup of flocs resulted in the formation of particles/flocculi and fine microflocs. Fluctuations in turbulence level were governed by seasonal variations in freshwater discharge and by the tidal cycle. During the wet season, strong freshwater input induced a high turbulent energy level that tended to generate sediment transfer from the coarser size classes (macroflocs, coarse microflocs) to finer ones (particles/flocculi and fine microflocs), and to promote a transport of sediment seawards. During the dry season, the influence of tides predominated. The turbulent energy level was then only episodically sufficiently high to generate transfer of sediment between floc size classes. At low turbulent energy, modifications in the proportions of floc size classes were due to differential settling. Tidal pumping produced a net upstream transport of sediment. Associated with the settling of sediment trapped in a near-bed layer at low turbulent energy, this causes the silting up of the waterways leading to the harbour of Haiphong.     

Tectonic influences on the Quaternary drainage evolution on the north-western margin of the French Central Massif: The Creuse valley example

This work examines the links between tectonics and fluvial dynamics on the north-western margin of the French Central Massif. Geomorphological and sedimentological analyses of detrital deposits were carried out as the basis for correlating the different formations, and for reconstructing the palaeodrainage in the Creuse basin. Cross-sections of the valleys, longitudinal profiles of terraces and glacis indicate post-depositional deformation of about 50 m amplitude: uplift north and south of Guéret and in the Éguzon district. These deformations are related to the Central Massif uplift of 50 m above the Paris Basin since 1.1 Ma (OIS 32). The Creuse incision varies from 140 m in the Crozant anticline to 60 m in the Paris Basin. An Upper Pliocene palaeochannel located on the Sédelle–Ardentes–Issoudun lineament is evidenced by andalusite coming from the Fougères unit. The reactivation of the transverse faults between Le Pin and Le Menoux has distorted the middle terraces. The main knick points are mainly due to tectonics and have been persistent in the landscape since the Upper Pliocene. Their recession rate is controlled both by discharge and lithology. Tributary streams exhibit strong convexities, indicating that the erosional response to Pliocene uplift has not yet propagated into upland surfaces.     

Messinian deposits and erosion in northern Tunisia: inferences on Strait of Sicily during the Messinian Salinity Crisis

Outcrops, offshore wells, electric logs and seismic profiles from northern Tunisia provide an opportunity to decipher the Messinian Salinity Crisis in the Strait of Sicily. Messinian deposits (including gypsum beds) near the Tellian Range reveal two successive subaerial erosional surfaces overlain by breccias and marine Zanclean clays, respectively. In the Gulf of Tunis, Messinian thick evaporites (mostly halite) are strongly eroded by a fluvial canyon infilled with Zanclean clays. The first erosional phase is referred to the intra-Messinian tectonic phase and is analogous to that found in Sicily. The second phase corresponds to the Messinian Erosional Surface that postdates the marginal evaporites, to which the entire Sicilian evaporitic series must refer. The Western and Eastern Mediterranean basins were separated during deposition of the central evaporites.     

Platinum-group element micronuggets and refertilization process in Lherz orogenic peridotite (northeastern Pyrenees,France)

Highly siderophile elements (Platinum-group elements, Au and Re) are currently assumed to reside inside base metal sulfides (BMS) in the convecting upper mantle. However, fertile lherzolites sampled by Pyrenean orogenic peridotite massifs are unexpectedly rich in 0.5–3 µm large micronuggets of platinum-group minerals (PGM). Among those, sulfides from the laurite-erlichmanite series (Ru, Os(Ir)S(As)₂), Pt–Ir–Os alloys and Pt–Pd–Te–Bi phases (moncheite–merenskyite) are predominant. Not only the BMS phases but also the PGM micronuggets must be taken into account in calculation of the PGE budget of orogenic fertile lherzolites. Laurite is a good candidate for equilibrating the whole-rock budget of Os, Ir and Ru while accounting for supra-chondritic Ru/Ir_N. Textural relationships between PGMs and BMS highlight heterogeneous mixing between refractory PGMs (laurite/Pt–Ir–Os alloys) inherited from ancient refractory lithospheric mantle and late-magmatic metasomatic sulfides precipitated from tholeiitic melts. “Low-temperature” PGMs, especially Pt–Pd bismuthotellurides should be added to the list of mineral indicators of lithosphere refertilization process. Now disseminated within fertile lherzolites, “lithospheric“ PGMs likely account for local preservation of ancient Os model ages (up to 2 Ga) detected in BMS by in-situ isotopic analyses. These PGMs also question the reliability of orogenic lherzolites for estimating the PGE signature of the Primitive Silicate Earth.     

Neubenennungen lithostratigraphischer Einheiten in der Helvetischen Kreide

In September 2007 Föllmi and coauthors requested the approval of four lithostratigraphic terms of the Swiss Committee of Stratigraphy (SCS). These terms were to be used in their publication on the Cretaceous in the Helvetic realm (Föllmi et al. 2007). At its meeting on October 18, 2007, the SCS decided the following: The Tierwis Formation (replacing the Drusberg Formation) was accepted, with the reserve that a section at Tierwis should be published in the near future. For the former “Lower Orbitolina Beds” the term Rawil Member was accepted, but here too a better type section than the one at Rawil Pass should be published. The term Rohrbachstein bed should not be used as a formal unit because it describes only a minor lithologic variation within the Grünten Member. The term Plaine Morte bed for a thin condensed horizon can not be accepted due to the fact that its definition is based mainly on biostratigraphy. Furthermore, the duplication a locality term which is in use for a previously established lithostratigraphic unit should be avoided.