Showing papers in "Bulletin De La Societe Geologique De France in 2001"

Tectonometamorphic evolution of the Schistes Lustres Complex; implications for the exhumation of HP and UHP rocks in the Western Alps

Abstract: We present new structural and metamorphic data on the Schistes Lustres complex which occupies a central position in the western Alps between the external LP and the internal UHP domains (e.g., the Dora Maira massif). Metamorphic conditions are shown to increase progressively from west to east from ca. 12-13 kbar/300-350 degrees C to 20-21 kbar/450-500 degrees C close to the Dora Maira massif. Two distinct exhumation episodes are recognized: (1) A pervasive east-vergent ductile D2 event, with a large component of vertical shortening, took place under low blueschist-facies to greenschist-facies conditions. This event is responsible for most of the exhumation of the Schistes Lustres complex as well as for the preservation of carpholite occurrences at its front, and took place during the period 50-40 Ma. (2) A west-vergent ductile-to-brittle, highly non-coaxial, extensional D3 event subsequently developed, with a deformation intensity decreasing from east to west. This event took place at subgreenschist-facies conditions and is coeval with (and probably derives from) the west-vergent greenschist deformation taking place in the Dora Maira massif and other internal domains by ca. 40-35 Ma.

The Rif mountain building (Morocco); a new tectonic scenario

Abstract: The building of the Alpine Rif belt (southern limb of the Betic-Rif orocline) is restored, mostly based on the Tertiary stratigraphic and metamorphic data set. The Betic-Rif Internal zones derive from an exotic Alboran Terrane partly involved in a S-dipping Betic subduction during the Late Cretaceous ?-Eocene. Incipient collision of the terrane against Iberia triggered back-thrust tectonics south of the Internal mountain belt during the latest Eocene-Oligocene. A N-dipping Maghrebian subduction developed from that time up to Middle Miocene, responsible for the rifting of the internal Alboran Terrane. Docking of the extending Alboran Terrane onto the North African margin occurred during the Neogene through the closure of the Maghrebian Flysch oceanic trough, with southwestward growth of the external accretionary prism, and foredeep subsidence. Subduction zone westward roll back associated with delamination of the dense lithosphere seem to account for the Betic-Rif late orogenic evolution.

The formation of the West European Rift; a new model as exemplified by the Massif Central area

Abstract: In this paper, we use mainly field data from the Massif Central area, which have been presented in a com­panion paper (Michon and Merle, 2001), to discuss the origin and the evolution of the West European Rift system. It is shown that the tectonic event in the Tertiary is two-stage. The overall geological evolution reveal a tectonic paradoxe as the first stage strongly suggests passive rifting, whereas the second stage displays the first stage of active rifting. ln the North, crustal thinning, graben formation and sedimentation at sea level without volcanism during the Lower Oligo­cene, followed by scattered volcanism in a thinned area during Upper Oligocene and Lower Miocene, represent the classical evolution of a rift resulting from extensional stresses within the lithosphere (i.e. passive rifting). In the South, thinning of the lithospheric mantle associated with doming and volcanism in the Upper Miocene, together with the lack of crustal thinning, may be easily interpreted in terms of the first stage of active rifting due to the ascent of a mantle plume. This active rifting process would have been inhibited before stretching of the crust, as asthenospheric rise associated with uplift and volcanism are the only tectonic events observed. The diachronism of these two events is emphasized by two clearly distinct orientations of crustal thinning in the north and mantle lithospheric thinning in the south. To understand this tectonic paradox, a new model is discussed taking into account the Tertiary evolution of the Alpine chain. lt is shown that the formation of a deep lithospheric root may have important mechanical consequences on the adjacent lithosphere. The downward gravitational force acting on the descending slab may induce coeval exten­sion in the surrounding lithosphere. This could trigger graben formation and laguno-marine sedimentation at sea level followed by volcanism as expected for passive rifting. Concurrently, the descending lithospheric flow induces a flow pattern in the asthenosphere which can bring up hot mantle to the base of the adjacent lithosphere. Slow thermal ero­sion of the base of the lithosphere may lead to a late-stage volcanism and uplift as expected for active rifting.

The evolution of the Massif Central Rift; spatio-temporal distribution of the volcanism

Abstract: The Massif Central area is the largest magmatic province of the West-European Rift system.The spa­tial-temporal distribution of Tertiary-Quaternary volcanism in the Massif Central, France, shows that three magmatic phases can be defined, each of them characterized by different volumes and different locations. The first event, termed the pre-rift magmatic event, is very scarce and restricted to the north of the Massif Central. It is suggested that this could result from lithospheric bending of the European lithosphere ahead of the incipient Alpine chain during the Pa­leocene. The second event, termed the rift-related magmatic event, is located in the north of the Massif Central only and is spatially connected with zones of high crustal thinning (i.e. the Limagne graben). It immediately follows Oligo­cene graben formation and associated sedimentation, and is represented by more than 200 scattered monogenic edifices. This second event can be attributed to partial melting as a consequence of lithospheric thinning that affected the north of the Massif Central during the rifting event. The lack of volcanism in the south during the same period of time is probably related to the very slight lithospheric thinning during the Oligocene. The third event, termed the major magmatic event, started first in the South in the upper Miocene at about 15 Ma, well after the end of the sedimentation. lt is unrelated to any extensional event. This major magmatic event reached the North of the Massif Central at about 3.5 Ma, following a pause in volcanism of about 6 Ma after the rift-related magmatic event. These two episodes of the ma­jor magmatic event are spatially and temporally associated with the two main periods of uplift, suggesting a common origin of volcanism and uplift processes. The major magmatic event can be attributed to late thermal erosion of the base of the lithosphere above a mantle diapir, as suggested by seismic tomography data. This general magmatic evolution drawn from data at the Massif Central scale may apply to the Eger graben as well, as the three magmatic events described in this study (pre-rift magmatic event, rifting event and post-Miocene volcanic event) are also reported in the literature. This suggests that a single cause should explain the formation of the entire western European rift surroun­ding the Alpine mountain belt.

Wind blown sediment mass budget of Sahelian village land units in Niger

Abstract: To assess the mass budget of aeolian sediments transported by wind (erosion vs. deposition) at the scale of village land units (25 kmX25 km), measurements were carried out during 3 years (from 1996 to 1998) in a cultivated field and in a fallow area simultaneously. These were located in the Sahelian zone of Niger with an average annual rainfall of 560 mm. The vertical upward fluxes of particles <20 mu m exported from the study area were estimated from the horizontal sediment fluxes measured using BSNE sand catchers. This mass of exported dust was compared with the vertical downward fluxes of particles of the same size range (<20 mu m) measured using passive CAPYR collectors. Values of deposition recorded in the field and in the fallow were similar. In the field, wind erosion reached its maximum in May and June when the vegetation cover was minimal. In the fallow area, wind erosion was always very low in comparison with the field. It occurred during the strongest storms when the grass cover was minimal. Nevertheless, the net balance between deposition and erosion was highly positive in the fallow areas. These results have been extrapolated at the scale of the village land units based on the current land use. At this scale, the balance was positive for the arable land, indicating a net deposition of aeolian sediments of +0.36 t ha (super -1) yr (super -1) . However, the complete disappearance of fallow land would result in a balanced budget for the arable land.

The oldest and first nearly complete skeleton of a chelid, of the Hydromedusa sub-group (Chelidae, Pleurodira), from the Upper Cretaceous of Patagonia

Abstract: A new pleurodiran (side necked) turtle is described on material from the Upper Cretaceous of Patagonia, from sediments outcropping at Cerro Blanco, Yaminue Creek, Rio Negro, Argentina. The sediments are compared to those from the Pellegrini lake area referred to the middle Member of the Allen Formation, Upper Campanian-Lower Maastrichtian. Yaminuechelys gasparinii n.g., n.sp., is a pleurodiran turtle on the pelvis sutured to the shell and a chelid on the formula of cervical vertebrae and the lateral cheek emargination, deeply extended towards (as here) or up to the posterior emargination. It is the oldest record of a nearly complete skeleton of a chelid, long necked (elongated cervical vertebrae, lowered skull), and the first sufficiently known of the Chelodina-Hydromedusa group (elongated skull, lowered neural arch and centrum of the cervicals, low zygapophyses processes, strong polygoned decoration) and of the Hydromedusa sub-group (widened inner nares by reduced palatine ossification). The carapace is 41,8 cm long. It is more primitive than Hydromedusa (Eocene-Extant, South America) and retains primitive characters either still present or no more present in the other chelids of the Pseudemydura, Emydura and Phrynops groups (short necked) and Chelus group (long necked), representing the anterior clades of phyletic diversification [Gaffney, 1977], or evolutive grades, of the family. Such are plesiomorphic, relative to Hydromedusa, the less pronounced lateral skull emargination, wider and longer hyoid elements, wider nucal and cervical, this not drawn back, presence of lateral mesoplastra, not shortened bridge, straight borders of the not shortened and not widened posterior plastral lobe, amphicoelous sacrals and caudal vertebrae uniting amphicoelous, concavoplaty--(i.e. anteriorly concave, posteriorly flat) and procoelous or weakly procoelous elements. As Hydromedusa, Yaminuechelys n. g. retains primitive characters such as the long series of neurals, the very lateral attachment of the axillar and inguinal processes and the attachment of the pelvis, below pleural 8 (and 7 in the extant form) and a small part of the suprapygal, and the ischitatic sutures prolonged on the xiphiplastral points. It is distinguished by the apomorphic presence of a wide and week anterior carapacial notch. Yaminuechelys n.g., or aff. Yaminuechelys spp. are known in Patagonia by fragmentary remains in a dozen of Upper Cretaceous and two Palaeocene localities. Before them, chelids are known in the world only by undefined smaller forms from Lower Albian and Upper Albian-Cenomanian Patagonian localities. In Australia, they are known from Palaeocene-Lower Eocene (no Cretaceous data before) with already extant Australian diversified forms. Yaminuechelys n.g. demonstrates how long the diversification in chelids is realized in South Gondwana before the full break of the continents.

Stratigraphie sequentielle du Dinantien type (Belgique) et correlation avec le Nord de la France (Boulonnais, Avesnois)

Abstract: The relative influences of local tectonics and global eustasy in the architecture of the sedimentary units of the Namur-Dinant Basin (southern Belgium) are determined. Nine third-order sequences are recognised. During the Lower Tournaisian (Hastarian and lower Ivorian) a homoclinal ramp extended from southern Belgium through southern England (Mendips) and into southern Ireland. From the upper Ivorian to the lower Visean rapid facies changes occurred due to progradation and increasing prominence of Waulsortian mudmounds. Progradation gradually produced a situation in which inner shelf facies covered the Namur (NSA), Condroz (CSA) and southern Avesnes (ASA) sedimentation areas, whereas outer shelf facies were restricted to the Dinant sedimentation area (DSA). During the middle and late Viscan a broad shelf was established from western Germany to southern Ireland. Because the shelf built up mainly by aggradation, parasequences can be followed over a large area. An early phase of Variscan shortening is perceptible during the Livian. The stratigraphic gap between the first Namurian sediments (E2 Goniatite Zone) and the underlying Visean varies from place to place, but is more important in the north. Sequence 1 straddles the Devonian-Carboniferous boundary. It starts with a transgressive system tract (TST) corresponding to the Etroeungt Formation (Fm.) and its lateral equivalent (the upper part of the Comb lain-au-Pont Fin.), and to the lower member of the Hastiere Fin. The highstand system tract (HST) is represented by the middle member of the Hastiere Fin. which directly overlies Famennian silicielastics in the northern part of the NSA. Sequence 2 starts abruptly, in the DSA and CSA, with the upper member of the Hastiere Fin. as the TST. The maximum flooding surface (MFS) lies within the shales of the Pont d'Arcole Fin., whereas the thick-bedded crinoidal limestones of the Landelies Fm. form the HST. Sequence 3 can clearly be recognised in the DSA and CSA. Its TST is formed by the Maurenne Fm. and the Yvoir Fm. in the northern part of the DSA and by the Maurenne Fm. and the Bayard Fin. in the southern part of the DSA. The Ourthe Fin. represents the HST. Growth of the Waulsortian mudmounds started during the TST. Sequence 4 shows a significant change of architecture. The TST is represented by the Martinrive Fm. in the CSA and the lower part of the Leffe Fin. in the DSA. The HST is marked by the crinoidal rudstones of the Flemalle Member (Mbr.) and the overlying oolitic limestones of the Avins Mbr. (respectively lower and upper parts of the Longpre Fin.). These latter units prograded far southwards, producing a clinoform profile. Sequence 5 is only present in the DSA and in the Vise sedimentation area (VSA). The TST and the HST form most of the Sovet Fm. and its equivalents to the south, namely, the upper part of the Leffe Fm. and the overlying Molignee Fm. In the VSA, the HST is locally represented by massive grainstones. Sequence 6 filled the topographic irregularities inherited from previous sedimentation. In the CSA, NSA and ASA the TST is formed by the peritidal limestones of the Terwagne Fm. which rests abruptly on the underlying Avins Nibr. (sequence 4) with local karst development. In the DSA, the TST corresponds to the Salet Fin. and, further south, to the black limestones of the strongly diachronous Molignee Fin. Over the whole Namur-Dinant Basin, the sequence ends with the thick-bedded packstones and grainstones of the Neffe Frn. as the HST. Sequence 7 includes the Lives Fm. and the lower part of the Grands-Malades Fm. (Seilles Mbr. and its lateral equivalents), corresponding respectively to the TST and HST. Sequence 8 corresponds to the Bay-Bonnet Mbr. (TST), characterised by stromatolitic limestones. The HST corresponds to the Thon-Samson Mbr. Sequence 9 is the youngest sequence of the Belgian Dinantian in the CSA and DSA. It includes the Poilvache Nibr. (TST, Bonne Fm.) and the Anhee Fm. (HST). These units are composed of shallowing-upward parasequences. The uppermost Visean and basal Namurian are lacking in southern Belgium where sequence 9 is directly capped by Namurian E2 silicielastics. In the VSA, sequence 9 is well developed.

Sismotectonique des Pyrenees; principales contraintes sismologiques

Abstract: This paper presents a critical review of the main seismological data which constrain the Pyrenean tectonics, mostly the spatial distribution of the hypocenters and the fault plane solutions. A new seismicity map is computed for the period 1989-99, and a compilation of the published focal solutions is presented and discussed. The Pyrenean range results from the convergence of the Iberian and Eurasian plates for about 65 Ma. This convergence followed a period of extension (-115 to -80 Ma), which is related to the opening of the Bay of Biscay. The boundary between the two plates, the North Pyrenean Fault (NPF), is recognized as a major tectonic feature running E-W to the north of the range. The eastern part of the range has been later affected by the Mediterranean tectonics (about 30 Ma ago), which generated some faults with NE-SW and NW-SE orientations in Catalonia. The Pyrenean structure is rather well known thanks to systematic studies carried out during the last two decades: crustal exploration from refraction and reflection seismic experiments, gravimetric studies, and tomographic imaging of the lithosphere using either seismological methods or magnetotelluric methods. The most important results are the detection of a 10-15 km Moho jump at the NPF with a thicker crust at the Iberian side, the evidence for a subduction of the Iberian lower crust beneath the North Pyrenean Zone along the eastern and central parts of the range, and the imaging, in the central and western Pyrenees, of two large blocks of lower crust material uplifted through the upper crust. Since 1989, several seismic networks set up by various institutions in the different parts of the range have allowed to produce a rather accurate seismicity map. Since 1997, the seismic network on the French side has been significantly upgraded, and the geographic distribution of the stations has been made more even. This network consists now in fifteen three-component, short period stations with telephonic transmissions, and five one-component stations with Meteosat satellite transmissions, which ensure an access to the data in case of damaging earthquake. Four stations of the French Atomic Energy Commission (CEA) are running in addition on the French side. 22 stations are set up on the Spanish side, with a higher density in Catalonia. With all these stations together, about 450 earthquakes with magnitudes > or =1.0 can be located each year in the Pyrenees, with a mean accuracy of + or -1.5 km for the horizontal coordinates, + or -3 km for depth, i.e. a factor of two better than with the previous networks. The new seismicity map reveals some new features which were not apparent in the previous map. In particular, the NPF is not a single linear feature, but it includes a multiplicity of segments which are not necessarily E-W oriented. The Adour Fault (also called "Bigorre Fault" in some previous articles), a NW-SE oriented fault in central Pyrenees, appears as a complex zone which possibly includes several segments parallel to each others. The seismicity beneath the Maladeta, which is the largest unit of the Paleozoic Axial Zone, appears more linearly distributed than in the previous map. Most of the events are located inside the upper crust (depth less than 12-15 km). Deeper events are mostly located in the western part of the range.

La rotation miocène inférieur du bloc corso-sarde. Nouvelles contraintes paléomagnétiques sur la fin du mouvement.

Abstract: The paleomagnetic investigations carried out in the 70's on Oligo-Miocene volcanics of Sardinia have demonstrated that the island was turned by 35-30 degrees clockwise from 33 Ma up to 3-1-20.5 Ma and rotated counterclockwise in a few million years [De Jong et al., 1969, 1973; Bobier et Coulon, 1970; Coulon et al., 1974; Manzoni, 1974, 1975; Bellon rr nl.. 1977: Edel et Lortscher, 1977; Edel, 1979, 1980]. Since then, the end of the rotation fixed at 19 Ma by Montigny er al. [1981] was the subject of discussions and several studies associating paleomagnetism and radiometric dating were undertaken [Assorgia er al., 1994: Vigliotti et Langenheim, 1995: Deino et al., 1997; Gattacceca rt Deino, 1999]. This is a contribution to this debate that is hampered by thr important secular variation recorded in the volcanics. The only way to get our of this problem is to sample series of successive flows as completely as possible, and to reduce the effect of secular variation by the calculation of means. Sampling was performed north of Bonorva in 5 pyroclastic flows that belong to the upper ignimbritic series SI2 according to Coulon rr nl. [1974] or LBLS according to Assorgia et al. [1997] (fig. I). Ar-40/Ar-39 dating of biotites from the debris flow (MDF) has yielded an age or 18.35 +/- 0.03 Ma [Dubois, 2000]. Five of the investigated sites are located beneath the debris flow ITV, TVB, TVD, SPM85, SPM86), one site was cured in the matrix of the debris flow (MDF) and one in 4 metric blocks included in the flow (DFC). Another site was sampled in the upper ash flow (PDM) that marks the end of the pyroclastic activity, just before the marine transgression. According to micropaleontological and radiometric dating this transgression has occurred between 18.35 and 17.6 Ma [Dubois, 2000]. After removal of a soft viscous component, the thermal demagnetization generally shows a univectorial behaviour of the remanent magnetization (fig. 2a). The maximum unblocking temperatures of 580-620 degrees (tab. I) and a rapid saturation below 100 mT (fig. 3) indicate that the carrier of the characteristic magnetization is magnetite. The exception comes: from the upper site PDM in which were found two characteristic components, one with a normal polarity and low unblocking temperatures up to 350 degreesC and one with a reversed polarity and maximum unblocking temperatures at 580-600 degreesC of magnetite. After calculation of a mean direction for each flow, the mean > direction 4 degrees /57 degrees (alpha (95) = 13 degrees) computed with the mean directions for the 5 flows may be considered as weakly affected by secular variation. But the results require a more careful examination. The declinations are N to NNW beneath the debris flow. NNW in the debris flow. and NNE (or SSW) above the debris flow, The elongated distribution of the directions obtained at sites TVB and TVD. scattered from the mean direction of TV to the mean direction of MDF is interpreted as due to partial overprinting during the debris How volcanic episode, The low temperature component PDMa is likely related to the alteration seen on thin sections and is also viewed as an overprint. As NNE/SSW directions occur as well below (mean direction << B >> : 5 degrees /58 degrees) as above the debris flow (PDMb : 200 degrees/-58 degrees). the NNW directions ( > : 337 degrees /64 degrees) associated with the debris flow volcanism may be interpreted as resulting from a magnetic field excursion. According to the polarity scale of Cande and Kent [1992, 1995] and the radiometric age of MDF, the directions with normal polarity (TV, TVB, TVD, SPM85. SPM86a. MDF. DFC) may represent the period 5En. while the directions with reversed polarity PDMb and SPM86b were likely acquired during the period 5Dr. Using the mean > direction, the mean >, or the PDM direction (tab. I). the deviation in declination with the direction of stable Europe 6.4 degrees /58.7 degrees (alpha (95) = 8 degrees) for a selection of 4 middle Tertiary poles by Besse et Courtillot [1991] or 7 degrees /56 degrees (alpha (95) = 3 degrees) for 19 poles listed by Edel [1980] can be considered as negligible. Using the results from the uppermost ignimbritic layer of Anglona also emplaced around 18.3 Ma [Odin rt al.. 1994]. the mean direction << E >> (3 degrees /51.5 degrees) leads to the same conclusion. On the contrary, when taking into account all dated results available for the period 5En (mean direction > 353 degrees /56 degrees for 45 sites) (tab. II). the deviation 13 degrees is much more significant. As the rotation of Sardinia started around 21-20.5 Ma. the assumption of a constant velocity of rotation and the deviations of the Sardinia directions with respect to the stable Europe direction locate the end of the motion between 18.3 and 17.2 or 16.7 Ma (fig. 4). During the interval 18.35-17.5 Ma, the marine transgression took place. At the same period a NE-SW shortening interpreted as resulting from the collision of Sardinia with Apulia affected different parts of the island [Letouzey et al., 1982]. Consequently, the new paleomagnetic results and the tectono-sedimentary evolution are in favour of an end of the rotation at 17.5-18 Ma.

Nouvelle interpretation des fractures des eruptions laterales de l'Etna; consequences pour son cadre tectonique

Abstract: Mt Etna is cut by numerous fractures (fissures and faults) of very different origin and orientation. They have been used to define the activity and the tectonic setting of the volcano. After a discussion of the proposed tectonic models for Etna, an examination of the fractures, which are linked to the high flank eruptions, was carried out based on the geological and geophysical studies of the recent eruptions (1983, 1989, 1991-93). All of these surface breaks are of strictly volcanic origin; they open and advance very slowly, in relation to the propagation of the dyke, as well as its width and depth from the volcano surface. If the dyke summit is not too far from the surface (about 200-300 m), fissures and normal faults, arranged in a graben, appear. When the dyke intersects the slope of the volcano, a flank eruption follows. Therefore, these fractures do not have a tectonic or volcano-tectonic origin: they do not cut the entire volcanic edifice, and thus cannot be used to define the rift-zones nor to characterise the tectonic regime controlling the functioning of Etna. They give information on the dyke orientation on the slopes of the volcanic edifice and cannot be used as significative markers of extension [Frazzetta and Villari, 1981; Kieffer 1983a and b; Monaco et al., 1997]. The simultaneous opening of radial fractures, according to various azimuths, is frequent and clearly indicates that, in these cases, the regional stress field is not implicated. But high on Etna, the concentration of flank eruptions, on the eastern side, and the orientation change of the fractures (fig. 6), when they travel away from the summit, have been repeatedly indicated. The repetition of flank eruptions and the azimuth changes can be explained, simply, by the closeness of the Valle del Bove [Murray, 1994], which induces a decrease of the confinement pressure. The dyke emplacements of the summit eruptions cause an eastward displacement of the higher part of Etna. Marine geophysical data indicate that this volcano is, however, not the site of a large scale lateral spreading to the Ionian sea. Consequently, an eastward detachment is present only on the superior part of the volcano (figs. 1B and 7C). In fact, an up to 100 m high and oversteepened east-facing scarp, between the towns of Vena and Presa, extends towards the south for some kilometers [Lanzafame et al., 2000]. It is made up of volcanic rocks affected by strong brecciation. Inverse faults are found in front of the scarp. The base of this one is found at the level of the pre-Etnean clays, which would have helped the displacement of the volcanics. The studies on the tectonic setting in which Etna is located has called the attention of numerous researchers. From the earliest studies, the presence of numerous normal faults has supported the idea that this volcano, as many others, is active in an extensional regime. The most recent geological and geophysical data show a more complex situation. Deep under Etna (more than 10 km), a compressive field (sigma 1 N-S) is present according to focal mechanisms [Cardaci et al.; 1990; Ferrucci et al., 1993; Cocina et al., 1997]. More superficially, instead, extension is usual. The importance of the weight of the volcanic edifice, in the spatial (horizontal and vertical) modification of the compressive stress field, must still be clarified. It is very clear, in any case, that Etna cannot be explained by an extensional regime or kinematics in extension [Monaco et al., 1997] using normal faults, which form during the flank eruptions.

La denudation tectonique de la zone ultradauphinoise et l'inversion du front brianconnais au sud-est du Pelvoux (Alpes occidentales); une dynamique miocene a actuelle

Abstract: In the western Alps, to the southeast of the Pelvoux massif (Champsaur-Embrunais-Brianconnais-Queyras transect), the Brianconnais zone consists of the southern tip of the Zone Houillere and small nappes of Mesozoic sediments, emplaced during the Eocene in HP-LT metamorphic conditions. During the Oligocene this tectonic pile was thrusted onto a late Eocene to early Oligocene flexural basin, deformed in low grade metamorphic conditions and belonging to the Ultradauphine zone. This major thrust, called here CBF [Chevauchement Brianconnais Frontal: Tricart 1986] represents the boundary between the external and the internal zones of the western Alps. It contains thin tectonic lenses of Subbrianconnais origin, so that the Brianconnais Front and the Penninic Front almost merge. Late Alpine extension. - We have recently discovered that the CBF was subsequently reactivated as an extensional detachment. This major negative inversion is associated with widespread extension in the internal (Brianconnais and Piemont) zones, resulting in multiscale normal faulting. Current field work in the Queyras area shows that this brittle multitrend extension is a continuation of the ductile extension that accompanied the exhumation of blue-schist bearing metamorphic units. Along the same transect, the external (Ultradauphine) zone was not affected by late-Alpine extension. This is still the present situation: to the east of the aseismic Pelvoux massif, the CBF bounds the Brianconnais seismic arc, the activity of which may be the continuation of the late-Alpine extension. At the scale of the western Alpine arc, active extensional-transtensional tectonics dominate in the internal zones while compressional uplift affects the external zone. In this contrasted stress field, the thrust-fault zone between internal and external arcs plays a major role of decoupling that can be demonstrated in several sites between the area analysed here and the Central Alps, including along the Ecors profile. Contribution of thermochronology. - In this paper, we compare apatite fission track (FT) ages from both sides of the inverted CBF to the southeast of the Pelvoux massif. In the hangingwall of the CBF, two ages were obtained from magmatic intrusions within the Zone houillere, close to Briancon. They are compared to recently published ages from the Champsaur Sandstones unit in the footwall of the CBF, along the same transect.

Mise au point d'un protocole experimental de separation granulometrique d'assemblages de nannofossiles calcaires; applications paleoecologiques et geochimiques

Abstract: Calcareous nannofossils play an important role in biostratigraphy and paleoecology, though their study is often complicated by their reduced size. The methodology presented herein allows to separate the nannofossil assemblage into granulometrically and taxonomically homogeneous fractions. This method permits :a) the concentration of rare taxa in order to simplify their morphometric study; b) the geochemical analysis (stable isotopes and trace elements) of these fractions. The latter is helpful in many cases, especially when the analysis of major or minor taxa constituent of the assemblage is greatly complicated by the reduced particles dimensions. In this way, it will be possible to bring light to the ecological parameters of these taxa, as calcification temperature and other ecological requirements (salinity, nutrient concentrations).

Les vallees fossiles de la baie de la Vilaine; nature et evolution du prisme sedimentaire cotier du Pleistocene armoricain

Abstract: The study of a dense network of high resolution seismic profiles in the bay of Vilaine, INSU-CNRS cruise Geovill, have led to the characterization of the architecture of the sediment wedge preserved between the coast and the 50 m isobath. This wedge lies on a substratum composed of three seismic units, U1, U2 and U3 respectively attributed to metamorphic and magmatic rocks, Lutetian and Ypresian sandy carbonates and post-Eocene sediments. The coastal sediment wedge comprises three major units. A basal unit (U4), dated around 600 to 300 ky BP, interpreted as braided river sandy conglomerates. A median unit (U5) corresponding to estuarine and fluvial sandstones and clays that give way to the west to mouth bar sandstones. A sommital unit (U6) attributed to marine argillites and barrier island sandstones dated from 8110+ or -200 years at the base. These three units are bounded by two major surfaces: an unconformity between U4 and U5 and a marine (wave and tidal) ravinement surface between U5 and U6. The unconformity is interpreted as a sequence boundary between two depositional sequences: a lower one with U4 seismic unit and a topmost one with U5 and U6 seismic units. Based on the available datations, the lower sequence is attributed to the Saalian and/or Elsterian glacial cycles and, the upper sequence to the Weichselian (lowstand systems tract) and to the Holocene marine transgression (transgressive systems tract). The passage from one sequence to the other corresponds however to a drastic shift in the paleoflow directions (60 degrees ) in the Bay of Vilaine closely related to the main faults orientations. The tectonic activity in Brittany during the Pleistocene, linked to intraplate stress, seems to exert a control on sediment architecture in the coastal wedge. Indeed, the tilt of the Armorican Massif during that period has caused a complete rejuvenation of the fluvial profiles in land and the separation of the paleo-Vilaine from the Paleo-Loire river courses.

Le filtrage des anomalies gravimetriques; une cle pour la comprehension des structures tectoniques du Boulonnais et de l'Artois (France)

Abstract: The geology of the Boulonnais has been well studied since the early part of the last century [Gosselet and Bertaut, 1873; Olry, 1904; Pruvost and Delepine, 1921]. Extensive coal exploration added substantially to the general understanding of the geology of the region but as outcrop is poor, many questions remain. Gravity methods used in the analysis of geological structures have had a long and successful history in helping to study the earth9s crust for scientific and applied objectives. Regional gravity data are particularly useful in mapping geographic distribution and configuration of density contrast of rocks. Previous gravity research shows the main trends of the structure. In most cases the regional Bouguer gravity hides the relationship between the geology and the shape of the anomaly caused by the perturbing body. New information can be obtained by filtering the maps. The purpose of filtering a map is to remove unwanted characteristics and enhance desirable characteristics that are diagnostic for the geology. Because of their simple mathematical forms, most potential field filters are in the spectral domain. It is advisable to transform the original unfiltered field to the spectral domain, apply the filter, then transform the filtered map back to the spatial domain for use in the interpretation. Several spectrally filtered versions of the original gravity map are used in this regional interpretation. In the case of the Boulonnais the most useful filters have been the horizontal component and the first vertical derivative. In the first instance computing the horizontal gradients of the gravity field permits us to localise the limit of the blocks and then the fault positions. The gravimetric field above a vertical contact of rock with different density shows a low on the side of the low density rocks and a high on the side of the high density rocks. The inflection point is located just on the contact of the two types of rocks. This contact can be outlined by locating the maxima of the horizontal gradient. In the case of a low dipping contact maxima stay close to the contact, but are displaced down dip. In the second instance the first vertical derivative acts as a booster for the short wavelength; this attenuates or destroys the effect of the regional field. The resulting map shows a better structure because in complex areas they give a better definition of the different bodies by separating their effects. In the case of the Boulonnais the first vertical derivative allows us to distinguish the depressed region from the uplifted one. The structural evolution of the Boulonnais-Artois area includes two main extensional events in the late Palaeozoic-early Cretaceous interval and an inversion in mid-late Palaeocene time. The new gravity data in combination with recent field and published data have provided a new insight into the structure of the Boulonnais-Artois area and a new interpretation is proposed. -- Fault patterns are oriented 110N and 040N in the Boulonnais and 140N in Artois areas. -- The linkage between the faults shows a relay geometry with transfer zones [cf. Morley et al., 1990 and Pea-cock and Sanderson, 1994]. The best example is located between Sangatte (near the tunnel) and Landrethun faults where overlapping synthetic faults with a relay ramp are imaged. -- There is no major continuous fault zone but a complex en echelon fault system. -- Linkage between Boulonnais and Artois fault is not well constrained. An important discontinuity between the two regions is apparent. This model underlines the importance of overlapping fault tips with the generation of transfer zones. These structures are also known in the Wessex and Weald basins [Stoneley, 1982; Chadwick, 1993] where heritage and inversion are significant.

The Middle Jurassic-Lower Cretaceous Rosso Ammonitico succession of Monte Inici (Trapanese Domain, western Sicily); sedimentology, biostratigraphy and isotope stratigraphy

Abstract: The pelagic Jurassic successions of the Trapani region of western Sicily are dominated by the characteristic Tethyan Rosso Ammonitico lithofacies. In order to revise the stratigraphy (sedimentology, microbiofacies, ammonite and calpionellid biostratigraphy, isotope stratigraphy) and to define a reference section, a well exposed succession of Rosso Ammonitico, spanning the Callovian-Upper Berriasian interval, has been selected in the eastern sector of Monte Inici, Trapani province. A sedimentological study has enabled us to distinguish three subfacies within the Rosso Ammonitico: nodular, pseudo-nodular, and intraclastic nodular subfacies. The two latter subfacies are characterized by the presence of intraclasts produced largely by mechanical abrasion, that suggest a formation by hydrodynamic processes. Increasing hydrodynamic energy is inferred from Lower Kimmeridgian to Upper Berriasian deposits. The vertical evolution of the microfacies shows the greatest skeletal diversity in the Middle Oxfordian, coinciding with a peak of delta 13 C. Ammonites are abundant in the Middle Oxfordian-Lower Kimmeridgian, where several ammonite zones have been recognized. With the aid of calpionellids we have been able to identify the Jurassic-Cretaceous boundary and to recognize high frequency biostratigraphic intervals in the Berriasian, that can be correlated throughout palaoetethys. The delta 13 C curve shows a marked positive peak at the base of the section (Lower to Middle Callovian) and a second, composite, positive excursion in the Middle-Upper Oxfordian. From the base of the Kimmeridgian, delta 13 C values slowly decrease, remaining low in the Berriasian. Oxygen isotope values exclude late burial diagenetic overprinting and the carbon peaks are interpreted as reflecting carbon cycle perturbations due to high biological productivity that may mirror global palaeoenvironmental variations. The Middle Oxfordian peak coincides with the highest skeletal diversity, with the first occurrence of the nodular facies, and with the onset of cherty limestones (lateral equivalents of radiolarites) in the southern sector of Monte Inici, and in localities to the west and north. The heteropy between Rosso Ammonitico and radiolarites s.l. suggests that sedimentation was controlled by local palaeotopography.

Metal ion geochemistry in smelter impacted soils and soil solutions

Abstract: As part of an extended project to understand the speciation of metal ions in contaminated lands, we conducted a series of chemical extraction experiments on soil and extracted the soil pore water to determine the speciation of Pb, Cd and Cu in a smelter impacted site. The chemical extractions show that soil organic matter and metal oxides (Fe and Mn) control the speciation of Pb, Cu and Cd in the soil. For Pb, these results are in agreement with previous EXAFS data on the same soils. The soil solution speciation is calculated with the help of the NICA-Donnan modelling approach. The modelling shows that Pb and Cu speciation is dominated by the dissolved organic matter while Cd is mainly in solution as a free aquo-ion. The speciation in the soil is also simulated by a model coupling both the binding to soil organic matter and metal oxides. The simulated partition coefficient (K d ) is in good agreement with the experimentally measured K d .

Stratigraphie séquentielle de l'Aptien du sous-bassin de Galvé (Province de Teruel, NE de l'Espagne)

Abstract: A correlation is established in a north-south transect based on continuous outcrops. Considering the different reference surfaces and the geometry, three major depositional sequences can be distinguished which can be subdivided into a complex arrangement of parasequences. These third-order sequences are composed of a lower retrogradational and an upper progradational trends. The first sequence contains orbitolinid bioaccumulations in the retrogradational trend and oolitic-bioclastic shoals in the progradational trend. The second sequence exhibits, from bottom to top, a transgressive, a regressive and a forced-regressive trends. Ammonite-rich marls characterise the transgressive trend, whereas bioconstructions rich in coral-chaetetids-microbialites are abundant in both regressive and forced-regressive trends. The maximum flooding of this sequence is widely distributed across the whole Iberian platform. Finally, the third sequence shows the installation of homogeneous rudistid bioaccumulations in a retrogradational and a progradational trends. Each major sequence boundary marks a community replacement, whose respective fossil associations are dominated by (1) orbitolinids, (2) corals-microbialites, (3) corals-chaetetids-microbialites, and (4) rudists.

Bioconstructions a spongiaires siliceux dans le Lias terminal du Bassin lusitanien (Portugal); stratigraphie, sedimentologie et signification paleogeographique

Abstract: The Upper Liassic series in the western border of Iberia (Lusitanian Basin, Portugal), show an important lutitic sedimentation, characterized generally by a monotonous marl/limestone alternation. Small scale siliceous sponge mudmounds occur in these deposits from Middle Toarcian to Lower Aalenian age. The scope of this work is to pinpoint the stratigraphical and sedimentological context and to characterize controlling factors of the spongioliths. Stratigraphic and facies analysis. Relevant sections were observed and investigated in different locations of the Lusitanian Basin (e.g., Alvaiazere, Porto de Mos, Rabacal, Coimbra and Cantanhede) (fig. 1). The siliceous sponge facies correspond to the upper part of the S. Giao Unit and to the lower part of the Povoa da Lomba Unit (fig. 2). Considering the sequential scheme of Duarte [1997], the sediments correspond to groups of third-order depositional sequences MST3 and MST4 (mainly in the upper part of this sequence: MST4B). The sedimentary evolution of these units shows a stacking pattern composed of shallowing upward sequences deposited in an outer homoclinal ramp setting, dipping northwestwards. Both units increase in thickness from south to north (fig. 3) and their vertical facies associations correspond to a very bioturbated (Chondrites, Zoophycos, Planolites and Thalassinoides) marl/limestone succession (figs. 4 and 5). MST3 is demonstrably more marly than MST4B. The base of MST4 [MST4A in Duarte, 1997] corresponds to a marl/marly limestone alternation, very poor in siliceous sponge mudmounds. The first unit (MST3) which includes sponge mudmounds is dated as uppermost Bifrons zone through the base of the Bonarellii zone. The majority of the siliceous sponge mudmounds occur within this time slice. These mounds are characterized by a great diversity of accompanying fauna mainly composed of brachiopods (rhynchonellids and terebratulids), crinoids and bivalves. The initial growth of the sponge build-ups can be correlated basin-wide to the intra Bifrons regional flooding surface (MST2/MST3 boundary). The second unit (MST4), particularly its upper part (MST4B), corresponds to the top of the Meneghinni-Opalinum interval and is related to a carbonate progradational phase. In the eastern part of the basin, the calcareous facies of MST4B are more bioclastic. Siliceous sponge mudmounds. The Toarcian mudmounds of the Lusitanian Basin are usually only a few decimetres thick and most display irregular knob-like to flat lenticular morphologies. Some build-ups are round and can reach 1,5 metres in thickness and ten metres in diametre. Also worth mentioning is a siliceous sponge biostrome developed at the base of MST3 in the Porto de Mos section (figs. 3 and 6). The upper mound surface is normally rough and uneven. In both sequences they are always related laterally with carbonate beds, which corresponds to the top of fourth order sequences. The mudmounds consist of mostly brownish iron-rich calcified siliceous sponges and a greyish, sometimes peloidal allochthonous micritic matrix. In general, the sponges themselves consist of dense leiolitic microbolites [automicrites sensu Reitner and Neuweiler, 1993]. The sponge spicules are diagenetically transformed into calcite. The great majority of the sponge specimens belong to the Hexactinosa (Class Hexactinellida) and are unknown and undescribed to date. "Lithistides" (polyphyletic desma-bearing demosponges) are very rare and only occur as forms encrusting Hexactinosan sponges. The benthic macrofauna is abundant and consists of monospecific crinoids, rhynchonellids, terebratulids and bivalves (mainly pectinids and ostreids). Encrusting organisms are serpulids, bryozoans and foraminifera, as well as "Lithistids" mentioned above. They are entirely restricted to the stratonomical surfaces of the siliceous sponges. The sponge bioherms consist of several microfacies types (wackestones, packstones, floatstones and boundstones). All of them are micrite dominated and represent low energy environments. They differ mainly in the amount of siliceous sponges, micrite, microbialites and the accompanying fauna. Palaeoenvironmental significance. The amount of microbial induced carbonate clearly mirrors the importance of microbial activity in respect of the reef building potential. Furthermore, three other controlling factors played an important role in the initiation of the siliceous sponge mudmounds of the Lusitanian Basin: bathymetry, sea-floor morphology and sedimentation rate. The role of the first two factors is evident because the siliceous sponge mudmounds are particularly important (abundance and volumetric expression) in the eastern part of the basin (Rabacal-Alvaiazere region). They are practically absent towards the west (essentially in MST4B) where the series show hemipelagic sedimentologic features (figs. 7 and 8). Reduced sedimentation rate is a precondition for the settlement of siliceous sponges and Hexactinosa in particular. Compared to all other Toarcian sequential units, MST3 and MST4B are the thinnest and therefore reflect the lowest sedimentation rates (fig. 8).

A new Lepisosteidae (Actinopterygii, Ginglymodi) from the Cretaceous of the Kem Kem Beds, southern Morocco

Abstract: Lepisosteids or gars constitute a very special neopterygian group, with seven living species in two genera: Lepisosteus and Atractosteus. They live in freshwaters from the eastern part of North America and Central America. A new lepisosteid, Oniichthys falipoui gen. nov., sp. nov., is described on the basis of two well preserved specimens. Although the type locality is unknown, information provided by the fossil collector, the type of preservation of the specimen, and the nature of the attached matrix indicate, with confidence, that it comes from the Kem Kem beds of southern Morocco (fig. 1). The Kem Kem beds are rich fossiliferous horizons, exposed along the face of an escarpment extending from the north of Erfoud town to the Kem Kem area. The age of these outcrops is still debatable being considered as ?Albian in age [Forey and Grande, 1998] or regarded as Cenomanian, due to their elasmobranch assemblage [Sereno et al., 1996]. Oniichthys falipoui shows several derived characters of gars such as an elongated ethmoid region, an upper jaw formed by a chain of tooth-bearing bones, a joint between the quadrate and the lower jaw lying far forward, in front of the orbit and a large splint-like quadratojugal overlying the horizontal branch of the preopercle. O. falipoui shares with the primitive gar, Obaichthys decoratus, from the ?Albian Santana Formation of Brazil, the presence of toothed maxillaries, although in the Moroccan taxon, the maxillaries are anteriorly fused with infraorbitals. This structure confirms that, at least, some of the "infraorbital chain" bones bear maxillary teeth, fused to them during ontogeny. Discussion of characters leads to regard O. falipoui as more derived than Obaichthys, and to place it as the sister-group of Lepisosteus-Atractosteus.

Petrologie du complexe alcalin sous-sature de Kokoumi (Cameroun)

Abstract: The Cameroon Line was created by the rejuvenation, at the beginning of the opening of the Atlantic Ocean, of a Pan-African N070 degrees E fracture zone [Moreau et al., 1987], which acted as a huge lithospheric crack taping a hot asthenospheric zone [Deruelle et al., 1998; Marzoli et al., 2000]. The Kokoumi anorogenic pluton belongs to the E-W Garoua rift structure, which represents the easternmost extension of the Benue trough. The Garoua rift opened during the Neocomian-Lower Aptian ages [Benkhelil, 1988] through the rejuvenation of Pan-African normal faults. The rift subsided, was partially filled by conglomerates and sandstones, and the ensemble was folded in the Cretaceous period [Guiraud, 1993]. Post-Cretaceous faulting affected these sediments. Intrusion of the Kokoumi anorogenic complex through the Cretaceous sandstones was favoured by N-S, N070 degrees E, E-W and N135 degrees E faults and N030 degrees E extension [Moreau et al., 1987]. The Kokoumi complex was first described by Koch [1959]. It is composed of a plutonic gabbro-nepheline monzosyenite-nepheline syenite series and of lamprophyric dykes (monchiquites and camptonites). One trachyte dyke is also observed. The gabbros are olivine (Fo 70 )-, nepheline-, or kaersutite-bearing gabbros. They also contain Ti-Al-rich diopside, Ti-rich biotite, titanite, ilmenite, Ti-magnetite and apatite. The nepheline monzosyenites contain diopside, Fe-diopside, kaersutite, Fe-kaersutite, titanite and apatite. The nepheline syenites contain aegirine-augite, F-rich arfvedsonite and aenigmatite. Kaersutite and clinopyroxene predominate in the lamprophyres. Monchiquites and gabbros, camptonites and monzosyenites, display respective similar mineralogy. Monchiquites contain carbonate ocelli. The trachyte does not contain ferromagnesian minerals. For gabbros and monchiquites, equilibrium Fe-Ti oxide temperatures are between 650 and 750 degrees C (+ or -40 degrees C) and oxygen fugacities between 10 (super -15) and 10 (super -14) (+ or -0.5 X 10 (super -15) ) atmospheres, according to Spencer and Lindsley [1981]. Nepheline crystallized below 700 degrees C, according to Hamilton [1961]. All the rocks (except the trachyte) are nepheline normative (Ne 6 to Ne 40 ). Major and trace element distributions in MgO-element diagrams for the two series merge together into a single trend, from monchiquites to nepheline syenites. Nevertheless, the monchiquites trends have different slopes. We deduce the evolution from gabbros to nepheline syenites on the one hand and from monchiquites to camptonites on the other from primitive mantle normalized multi-element diagrams. Multi-element diagrams for the trachyte and the nepheline syenite are strictly similar. Patterns for Kokoumi gabbros are similar to those for basalts of the Kapsiki plateau [Ngounouno et al., 2000] and the Garoua rift [Ngounouno et al., 1997] with typical negative K and positive Zr and Ti anomalies. Patterns for nepheline monzosyenites display negative anomalies in Sr, P, Eu and Ti and those for nepheline syenites and trachyte display greater anomalies in these elements and Ba. Compared to gabbros, nepheline monzosyenites are enriched in all REE with a concave upward pattern and no Eu-anomaly. Nepheline syenites have a range of broadly similar REE patterns to nepheline monzosyenites with steep slope from La to Sm, strong Eu negative anomaly (Eu/Eu (super *) nearly equal 0.15) and heavy-REE spoon-shape. REE patterns for monchiquites, camptonites, and trachyte are respectively similar to those for gabbros, monzosyenites, and nepheline syenite. Initial Sr-isotope ratios of 0.7033 (recalculated from the measured ratios for an age of 39 Ma for plutonic rocks and 20 Ma for the lamprophyres and the trachyte) are similar to those obtained for basalts from the continental segment of the Cameroon Line [Halliday et al., 1988; Ngounouno et al., 2000; Demaiffe et al., unpubl.], whereas nepheline syenites and trachyte are distinctly more radiogenic with values between 0.7128 and 0.7251. Amphibole and whole-rock K-Ar analyses (table III) yield 39.0+ or -0.9 Ma and 36.6+ or -0.9 Ma respectively. Since amphibole is a reliable chronometer in K-Ar dating, we propose the first age as the probable time of emplacement of the gabbros. Whole-rock analysis of nepheline syenite 99 displays an age of 33.1+ or -0.5 Ma. Field and geochemical observations suggest that gabbros and nepheline syenite are cogenetic and hence contemporaneous.

Mouvement post-messinien sur la faille de Nîmes: Implications pour la sismotectonique de la Provence

Abstract: The seismicity of southern France probably results from the convergence between Africa and Europe which proceeds at a rate of approximately 0,8 cm/year at the Provence longitude [Nuvell-DeMets et al., 1990]. The potentially active faults delimit a large panel in the Mesozoic cover. It includes E-W compressive structures (Mont-Ventoux, Montagne de Lure to the north, Luberon, Costes et Trevaresse to the south) and NE left-lateral strike slip (Durance to the east and Nimes, to the west, and possibly the Cevennes Fault) [e.g., Grellet et al., 1993; Sebrier et al., 1997; Lacassin et al., 1998]. The Nimes Fault, which is considered as one of the main faults of southeastern France [Combes, 1984; Grellet et al., 1993; Ghafiri, 1995] is associated with only few and small seismic events, but paleoseismic evidence for larger earthquakes, with magnitudes possibly as large as 6.5, were found on a subsidiary fault near Courthezon [Combes et al., 1993]. Here, we try to quantify fault activity over a longer period of time than that accessible from the usual geomorphic approach, by assessing possible displacement of Messinian markers on the Nimes fault. In the early Miocene a regional erosion surface of Burdigalian age (around-20 Ma) was formed. This surface is still preserved and has not been much deformed west of the Nimes fault. To the east, this surface is only gently folded due to E-W anticlines [Champion, 1999; Champion et al., 2000]. This contrast suggests that the Nimes fault has been active and has accommodated N-S shortening after the abandonment of the Burdigalian erosion surface. The Nimes and Pujaut faults can be followed in the topography between Nimes and Sauveterre where they are generally bounded by outcrops of Mesozoic limestone (fig. 2A-3). To the NE, the Nimes fault can be roughly traced across the Quaternary Rhone alluvium, between Sauveterre and the Mont Ventoux. It is marked by disruption of the continuity of the terrasses of Chateauneuf-du-Pape. The signal is only tenuous and cannot be used to infer precisely the fault location and segmentation but suggests that the faults have been active during the Quaternary. During the Messinian, starting at about -5.95 Ma, the Mediterranean sea level fell by about 1500 m [Clauzon, 1975; Krijgsman et al., 1999; Gautier et al., 1994; Cande et Kent, 1992-1995; Clauzon et al., 1995]. The major tributaries were forced to cut down and formed deep and narrow valleys. The Mediterranean sea rose up to an elevation of +80 m NGF at -5.32 Ma, flooding the canyons, and remained stable until about -3.8 Ma [Vail and Mitchum, 1979; Benson et al., 1991; Cita, 1975; Haq et al., 1987; Hilgen et Langeries, 1993]. After -5.32 Ma the canyons were filled with Pliocene sediments. The canyon formed by the Rhone incision during the Messinian crisis is well documented [Clauzon, 1982; Clauzon et al., 1995; Clauzon et al., 1999; Rubino et al., 2000]. We found evidence for a tributary canyon on an old seismic line ELF M2S8. The canyon strikes E-W between the "Barre de Roquemaure" and "Barre de Caderache" and should cross the Nimes Fault. In order to constrain more tightly its geometry near the Nimes Fault, we have implemented three seismic lines. If we trace the position of the southern border of the canyon using the different profiles and the surface geology, we find that the horizontal offset at the fault cannot be much larger than about 500 meters. In order to image a possible smaller offset we have determined the geometry of the canyon from a microseismic zoning technique [Nakamura, 1989; Duval et al., 1997; Ibs-von Seht M. and Wholenberg, 1999; Sabourault, 1999]. Measurements were conducted at 37 points which were used in complement. The depth to the canyon bottom was determined using the velocities derived from the seismic profiles and was cross-checked from the comparison with geological log at points F1 and F2. The geometry of the southern edge of the Messinian canyon, shows a left-lateral offset of 440 m + or -50 m, which might be taken to reflect post-Messinian fault motion. Assuming that the observed 440 m offset of the Messinian canyon at the Nimes fault is due to fault motion, and that the fault slip rate has not varied significantly since the Messinian crisis, we derive a left-lateral slip rate of 0.06 to 0.09 mm/year. Given that the observed deflection of the flank of the canyon might in part be of non tectonic origin, our study basically places an upper limit of 0.09 mm/yr on the slip rate on the Nimes faults. Such a low slip rate is comparable with estimates obtained on the Durance Fault [Baroux, 2000] and on the E-W folds and thrust faults east of the Nimes faults such as Ventoux-Lure and Alpilles-Costes-Trevaresse [Champion, 1999; Champion et al., 2000]. Although the details of the kinematics scheme of active deformation of Provence remain a matter of discussion, these various faults are probably linked and must have similar slip rates.

Nature des sources des composants andesitiques des Gres du Champsaur et des Gres de Taveyannaz; implications dans l'evolution des Alpes occidentales au Paleogene

Abstract: Paleogene magmatism is widespread in the western Alps and its origin is still a matter of debate. It consists of calc-alkaline and shoshonitic suites with mainly granodioritic intrusions in the Internal Zone (northern Italy), andesitic volcanism in the Delphino-Helvetic Zone (French-Swiss domain) and the Esterellite intrusions in Provence. In the External Alpine Zone, the Taveyannaz Sandstones and the Champsaur Sandstones preserve andesitic pebbles and mineral fragments dated at 32 Ma. On the basis of trace-element and isotopic geochemistry (Sr, Nd, Pb and O) of mineral separates (amphibole, pyroxene and plagioclase) and andesitic clasts, we characterise the nature of magmatic source(s) and compare the isotopic compositions of the andesites with those of well known Paleogene igneous suites.

L'excursion isotopique du carbone organique (delta 13 C org ) dans les paleoenvironnements continentaux de l'intervalle Paleocene/Eocene de Varangeville (Haute-Normandie)

Abstract: The late Palaeocene carbon isotope excursion (C.I.E.) is often regarded as the best means of correlating marine and continental deposits. The few isotopic studies carried out in continental environments were based on pedogenic carbonate [Koch et al., 1992], or on organic matter. Sinha [1997] took up this subject starting from the outcrops on the coast of the English Channel at Varangeville, where marine sequences biostratigraphically constrain the isotopic excursion. His work documents a negative delta 13 C org excursion value approximately -27 per mil PDB. The present work points out that it is necessary to study more complete sections than those studied by Sinha. A section named Phare d'Ailly has been sampled in detail (figs. 1, 2, 3). In this section, for which detailed analysis of sedimentology, palaeontology and organic matter facies establish the continental nature of the palaeoenvironment, isotopic analysis of organic matter reveals a very negative delta 13 C org excursion value approximately -30 per mil PDB. The P/E interval synthesized in figure 2 shows three main groups, the stratigraphy of which is strongly constrained between the calcareous nannofossil Zones NP8 and NP11. The marine Thanetian facies belong to Zones NP8 and NP9. Above, the "Sparnacian" (Mont Bernon Group) is divided into 5 units referred to as SP. Unit SP2 is attributed to the Peckichara disermas Charozone, equivalent to Zone NP9. For unit SP4, an indirect correlation with Zone NP10 may be deduced. The upper part of the Varangeville Formation is known for its nannofossil association attributed to Zone NP11. We may conclude from these observations that the sparnacian sediments are synchronous with NP9-10 Zones and that they are effectively located in the time interval of the delta 13 C excursion. The SP1 and SP2 sediments were analysed for carbonate content, grain size distribution, clay mineralogy and total organic content (T.O.C.), using standard laboratory methods (fig. 3). An optical specification of the organic matter has been obtained from semiquantitative analysis of the organic matter facies. The survey of macro- and microfossils has supplied complementary supports for isotopic analysis (seed, wood and charcoal). The isotopic measurements were carried out on the bulk sediment and on complementary supports with a mass spectrometer in continuous flow delta + (Finnigan Mat). The measurement precision is better than 0.1 per mil and the reproducibility is about 0.15 per mil. Washing residues provide Gasteropoda, Bivalvia, Ostracoda, Characea, as well as fruits and seeds, microcodiums and otolith. All the palaeontological data illustrate a continental biota of a pure lacustrine environment, quiet and shallow, in a hot and humid climate. The organic matter facies analysis confirms the absence of any marine influence and documents a palaeoenvironment of lakes and ponds having an anoxic floor. Thus the carbon of the organic matter is considered to be continental and their isotopic variations linked to those of the atmospheric carbon.

Le marais estuarien de la Songolo (Sud Congo) a l'Holocene moyen et recent

Abstract: This paper presents the most recent and detailed Holocene palynological research carried out on the Congolese Atlantic littoral, in the region of Pointe-Noire, as part of the ECOFIT program (CNRS-IRD) The results are obtained on two continental sedimentary sequences (core S2 from Songolo site: 4 degrees 45'51"S, 11 degrees 51'55"E, alt 5 m and core 2 from Coraf site: 4 degrees 45'S, 11 degrees 51'E, alt 1 m) Pollen data and interpretation are completed on the Songolo sequence by mineralogical (quartz, kaolinite, TOM) and isotopic (delta 13 C on organic matter) analysis allowing a continuous reconstruction of the Congolese littoral palaeoenvironments from 7000 yr cal BP onwards The Congolese coastal plain is today a mosaic of forest-grassland Sandy open short savanna largely occurs with as dominant grass Loudetia arundinacea Locally, Symphonia globulifera swamp forests are present along rivers and in flooded areas Mangrove with Rhizophora is not well developed except along the Kouilou estuarine The climate is characterized by mean annual precipitation not exceeding 1300 mm and temperature ranging between 22 and 25 degrees C The core S2 from Songolo site was extracted using a piston corer The coring site is today a peat building swamp with abundant Cyperus papyrus The local dominant trees are Raphia and Alstonia The presence of Elaeis guineensis (oil palm) and Mangifera indica (mango) testifies of a recent human occupation of this area The core Coraf 2 was collected on the beach by manual penetration of PVC tube On the two studied cores, dating control is provided by thirteen 14 C conventional and AMS dates performed on total organic matter The ages are reported with equivalent ranges in calendar years The present pollen, mineralogical and isotopic records, completed by previous other data (mainly macroflora remains), give new informations on vegetation and hydrological changes at local and regional scale since 7000 yr cal BP on the Congolese littoral, in relation to climatic and sea level changes The main conclusions can be summarized as follows During the middle Holocene (7000-3000 yr cal BP), expansion of swamp dense forests with Pandanus, Anthostema, Hallea, Symphonia globulifera, Syzygium, and also of mangrove (Rhizophora), as testified by pollen analysis, gives evidence of high rainfall which is also supported at Songolo by the abundance of kaolinite and quartz, an increase in the amorphous silica and delta 13 C low values typical of C 3 biomasse until 3000 yr cal BP At the same time, the occurrence in macroflora remains of trees such as Saccoglottis gabonensis, Ongokea gore and several species of Monopetalanthus that are found today in the Mont de Cristal forest in Gabon, under 2000-2500 mm of annual rainfall, confirms this climatic interpretation A major change is registered ca 3000 yr cal BP, characterized by the regression of the swamp forests which are replaced by herbaceous formations dominated by Cyperaceae and ferns, the disappearance of mangrove near the Songolo site, an important decrease in mineral influx (quartz and kaolinite) and increase in organic matter content Increase in delta 13 C values during this period shows that most of the organic fraction derives from C 4 plants, probably linked to local expansion of Cyperus papyrus This change, well known at this time on several sites from Central Atlantic Africa, is mainly linked to a progressive regional deterioration of climate towards aridity On the Songolo site, the presence of fresh water taxa such as Nymphaea lotus supports an absence of marine water influence, indicating a sea level regressive episode well known, at the same time, along the West African coast During the most recent period, despite more favourable climatic conditions (rainfall), no re-extension of littoral swamp forests is detected These results are mainly related to the location of the studied sites, in an area of particularly intense human impact as shown by the occurrence of Elaeis guineensis (oil palm) in macroflora remains recovered from numerous archeological sites The multidisciplinary data presented here correlate well with previous records from Central Atlantic Africa (Congo and Cameroon) The general similarity of the registered changes during the last 7000 yr cal BP strongly supports a regional climatic interpretation

Paragenese a faible variance dans les metapelites de la serie de Filali (Rif interne marocain); description, interpretation et consequence geodynamique

Abstract: The metamorphic series of Filali and the Beni Bousera massif represent the most metamorphic unit in the inner part of the Moroccan Rif. The Filali series is composed of micaschists, gneisses, migmatites and granulites wrapped around the ultramafic body of the Beni Bousera. The foliation is broadly coherent all over the massif although differences in lineation and fold axis have been found between the granulites and the micaschists-gneisses. Metamorphic grade increases continuously towards the ultramafic body; however, seven metamorphic zones which account for this increase can be defined: - chlorite zone: muscovite and chlorite underline the S1 schistosity refolded by S2. Small garnet is wrapped by or cut across S2; - chloritoid zone: chloritoid displays the same textures as chlorite and muscovite, underlining S1 and S2 schistosity. Chlorite reacts out to biotite; - staurolite+andalusite zone: staurolite, andalusite, cordierite and garnet have pre-, syn- and post- kinematic features with respect to S2. Garnet reacts out to muscovite, biotite and plagioclase; - staurolite+andalusite+kyanite zone: same textural relationships as in the previous zone. Kyanite is an additional phase, often displaying epitaxial textures with staurolite and andalusite; - andalusite+kyanite+sillimanite zone: fibrolitic sillimanite generally associated with biotite crystallizes from staurolite. However, sillimanite is also directly formed from andalusite and kyanite. As in the previous zone garnet reacts out to biotite, muscovite and plagioclase; - sillimanite+k-feldspar zone: muscovite reacts out to k-feldspar. Modes of biotite and sillimanite increase; - migmatites zone are composed by biotite, sillimanite, garnet, cordierite quartz and feldspars. Garnets do not display the reaction textures seen in the previous zones; - granulites are composed of a primary paragenesis with kyanite+k-feldspar+garnet+ or -biotite+plagioclase+rutile, partially obliterated by a secondary paragenesis with cordierite, sillimanite and spinel.

Tectonic evolution of the Cevennes para-autochthonous domain of the Hercynian French Massif Central and its bearing on ore deposits formation

Abstract: The Cevennes area belongs to the para-autochthonous domain of the Hercynian Belt of the French Massif Central. Three lithological series, namely: sandstone-pelite, black micaschist and gneiss-micaschist, are identified. They form an imbrication of five tectonic units which overthrust the unmetamorphosed Viganais Paleozoic units to the south and the gneissic Mamejean Unit to the north. The structural, metamorphic and magmatic evolution of the Cevennes area is characterized by three events, namely: (1) southward shearing coeval to a MP/MT metamorphism dated around 340 Ma; (2) post nappe anatexis (T 5 kb); (3) Namurian (ca 315 Ma) E-W extensional tectonics and plutonism. The structure of the Mt-Lozere-Borne granitic complex is constrained by new AMS and gravimetric data. The plutons are the driving power of the hydrothermal convective circulations responsible for an early deposition of diffuse arsenopyrite in the thermal aureole. Gold bearing sulfides are afterwards concentrated in quartz veins along brittle normal and wrench faults around the granite. Lastly, ore bearing quartz pebbles are sedimented in the Stephanian Ales coal basin.

Chronologie de l'ouverture du bassin des Ksour (Atlas Saharien, Algerie) au debut du Mesozoique

Abstract: The Ksour mountains (northwest Algeria) belong to the western part of the Saharan Atlas. They are located between the High Plateau to the north, the South Atlas Front to the south and the Moroccan High Atlas to the west (fig. 1a). During the last decades, the Ksour mountains were the focus of several studies, mainly based on sedimentary analyses [Cornet, 1952; Bassoulet, 1973; Baiche, 1975; Ameur, 1978; Ait Ouali, 1991; Mekahli, 1995; Elmi et al., 1998]. These studies indicate that the Ksour mountains correspond to an old asymmetric rift, trending NE-SW, which underwent extension during the Triassic and Liassic [Ait Ouali, 1991; Frizon de Lamotte et al., 2000]. In order to precise the timing of the rifting processes we have analysed subsurface data (seismic lines and borehole data) from the Sonatrach Company.

Existe-t-il une crise au passage Lias-Dogger chez les ammonites? Approche morphometrique par quantification de la disparite morphologique

Abstract: Morphological changes observed in ammonites at the Lias-Dogger (Toarcian-Aalenian) boundary are analyzed here. The study covers all known ammonites worldwide over some 2 m.y. (the last two biozones of the Toarcian: Pseudoradiosa and Aalensis and the first biozone of the Aalenian: Opalinum). Ammonite taxonomy at this boundary is still open to question, the main disagreements being over groupings at family or super-family level. The evolutionary schema developed by Tintant and Mouterde [1981] shows two main sets of Ammonitina for the time: (1) the dominant Hammatocerataceae (Graphoceratidae+Hammatoceratidae) and (2) the final representatives of the Hildocerataceae. Two conservative ammonite stocks coexisted alongside these Ammonitina: Phylloceratina and Lytoceratina. The ammonites under review are those that made the transition from the "typically Liassic stock" to the "post-Liassic stock" prefiguring all subsequent Jurassic ammonites. A morphometric procedure is used to quantify ammonite morphological disparity rather than taxonomic diversity. A set of 35 characters were considered, corresponding to the end of the phragmocone and to the body chamber. We selected 64 morphologies covering the entire range of shapes found at the subzone scale. Morphological disparity is expressed graphically by morphospaces (first factorial plane of correspondence analysis) and arithmetically by a space occupation index (sum of variances on the factor axes). The FAD and LAD of the selected species allow us to study morphospace changes at the subzone scale. The results clearly show changes occurring in the course of the seven subzones. The earliest subzone (Levesquei) has points weakly grouped around two poles (negative and positive F1 values) with an isolated point characterized by a very negative F2 value. Changes in this morphospace from the Pseudoradiosa subzone to the Lugdunensis subzone are marked by increased dispersion around the two poles. By contrast, an increasing number of morphologies have clearly negative F2 values. The Lias-Dogger boundary displays further dispersion of the two morphological poles, leading to them becoming indistinct. Continuation after the boundary shows a very similar pattern with shapes scattered and covering most of the morphospace. The morphospace changes analyzed previously were quantified by the sum of variances of the first 15 factorial axes, representing more than 90% of the information contained in the original matrix. For the Ammonoidea as a whole, the poorest coverage of the morphospace occurred in the Levesquei subzone, while the best coverage occurred in the Comptum and Bifidatum subzones. Between-times, morphological disparity increased at varying rates with no drastic changes at the Lias-Dogger boundary. A similar procedure was performed for the Ammonitina only. Indeed, the occurrence of two conservative sets (Lytoceratina and Phylloceratina) could be suspected of inducing -- at least initially -- two peculiar morphological poles and introducing a bias into the quantification. The results are similar, bringing out more clearly the increase in morphological disparity at the end of the Lias, and its stagnation thereafter. It is difficult to say why morphological disparity increased as it did. The main problem is the lack of any clearly resolved phylogeny for the ammonites under study. However, if we accept the hypothesis of Tintant and Mouterde [1981] recognizing five homogeneous clusters for ammonites of these times, it can be seen that two of them alone (Graphoceratidae first, and Hammatoceratidae later) account for the increased disparity. Within this phylogenetic assumption, it is improbable that external constraints alone, such as eustasy or physico-chemical phenomena, could have brought about the increase, for they would probably have affected all of the ammonoids, or at least the three Ammonitina groups that were initially quite similar in shape (in the Levesquei sub-zone). It is more likely then that internal factors (e.g. heterochrony) leading to two consecutives biological "explosions" (Graphoceratidae and Hammatoceratidae) were instrumental in bringing about this increased disparity. Finally, if the Lias-Dogger boundary is clearly marked by changes at any taxonomic level, our results--based on a quantification of morphological disparity and not on taxonomy--show that changes in ammonites can only be properly understood in a broader context: there was some morphological change at the Lias-Dogger boundary but it was initiated at the beginning of the Pseudoradiosa subzone. The Lias-Dogger event was thus not an unusual or major one, but more the end of a sustained event initiated two zones earlier. This period of time (final two zones of the Toarcian) witnessed progressive growth in disparity, which ceased only at the beginning of the Dogger.

Le magmatisme basique de la boutonniere d'Igherm (Anti-Atlas occidental, Maroc); un jalon des distensions neoproterozoiques sur la bordure nord du craton ouest-africain

Abstract: In the Igherm inlier (western Anti-Atlas, Morocco) doleritic dyke swarms with various directions and gabbroic intrusive bodies were emplaced during Neoproterozoic times, cutting across either Eburnean micaschists and granites or Panafrican limestones and quartzites. All these rocks were deformed by the main Panafrican schistosity and covered by molassic and volcanic Upper Neoproterozoic series. The primary mineralogical assemblages (plagioclase, augite, olivine...) of the mafic rocks are nearly completely replaced by secondary assemblages (albite, actinolite, chlorite, epidote, calcite, quartz, leucoxene, magnetite, hematite...). However, three main groups have been recognized by the means of relative chronology and petrography. The group 1 is earlier, as shown by the intrusive character of the dykes of the other two groups into its gabbroic bodies. Using incompatible trace elements and rare earth elements it appears that this magmatism is truly heterogeneous and that the three groups have different magmatic affinities. The group 1 corresponds to tholeiitic dolerites and gabbros characterized by intersertal and ophitic textures and by high contents in Fe 2 O 3 (12.16 to 16.64%), TiO 2 (1.46 to 2.5%), Zr (90 to 174 ppm), Nb (7 to 13 ppm), Y (21.68 to 38.74 ppm) and V (264 to 419 ppm). The REE contents are low (Sigma REE = 49 to 137 ppm) and the REE patterns are flat [1.99 Eu/Eu (super *) <1.11). These features as the TiO 2 vs FeO (super *) /MgO and V vs Ti/1000 diagrams are characteristic of anorogenic intraplate magmas. The group 2 corresponds to calc-alkaline dolerites and gabbros showing fine-grained intersertal textures and high contents of Al 2 O 3 (14.10 to 20.64%) and low contents of Fe 2 O 3 (8.35 to 12.91%), TiO 2 (0.68 to 1.41%), Zr (66 to 106 ppm), Nb (5 to 7 ppm), Y (16.41 to 20.75 ppm) and V (144 to 264 ppm). The REE contents vary from 67 to 155 ppm and the REE patterns are fractionated (2.78<(La/Yb) N <6.62) with a strong enrichment in LREE. The slight positive Eu anomaly (0.91