The significance of the Briançonnais domain in the Alpine orogen is reviewed in the light of data concerning its collision with the active Adriatic margin and the passive Helvetic margin. The Briançonnais which formerly belonged to the Iberian plate, was located on the northern margin of the Alpine Tethys (Liguro-Piémont ocean) since its opening in the early-Middle Jurassic. Together with the Iberian plate the Briançonnais terrane was separated from the European plate in the Late Jurassic-Early Cretaceous, following the northern Atlantic, Bay of Biscay, Valais ocean opening. This was accompanied by the onset of subduction along the northern margin of Adria and the closure of the Alpine Tethys. Stratigraphic and metamorphic data regarding this subduction and the geohistory of the Briançonnais allows the scenario of subduction-obduction processes during the Late Cretaceous-early Tertiary in the eastern and western Alps to be specified. HP-LT metamorphism record a long-lasting history of oceanic subduction-accretion, followed in the Middle Eocene by the incorporation of the Briançonnais as an exotic terrane into the accretionary prism. Middle to Late Eocene cooling ages of the Briançonnais basement and the presence of pelagic, anorogenic sedimentation lasting until the Middle Eocene on the Briançonnais preclude any sort of collision before that time between this domain and the active Adria margin or the Helvetic margin. This is confirmed by plate reconstructions constrained by magnetic anomalies in the Atlantic domain. Only a small percentage of the former Briançonnais domain was obducted, most of the crust and lithospheric roots were subducted. This applies also to domains formerly belonging to the southern Alpine Tethys margin (Austroalpine-inner Carpathian domain). It is proposed that there was a single Palaeogene subduction zone responsible for the Alpine orogen formation (from northern Spain to the East Carpathians), with the exception of a short-lived Late Cretaceous partial closure of the Valais ocean. Subduction in the western Tethyan domain originated during the closure of the Meliata ocean during the Jurassic incorporating the Austroalpine-Carpathian domain as terranes during the Cretaceous. The subduction zone propagated into the northern margin of Adria and then to the northern margin of the Iberian plate, where it gave birth to the Pyrenean-Provençal orogenic belt. This implies the absence of a separated Cretaceous subduction zone within the Austro-Carpathian Penninic ocean. Collision of Iberia with Europe forced the subduction to jump to the SE margin of Iberia in the Eocene, creating the Apenninic orogenic wedge and inverting the vergence of subduction from south-to north-directed.

No abstract

No abstract

No abstract

No abstract

Interactions and coordination between conspecific individuals have produced a remarkable variety of collective behaviours. This co-operation occurs in vertebrate and invertebrate animals and is well expressed in the group flight of birds, fish shoals and highly organized activities of social insects. How individuals interact and why they co-operate to constitute group-level patterns has been extensively studied in extant animals through a variety mechanistic, functional and theoretical approaches. Although collective and social behaviour evolved through natural selection over millions of years, its origin and early history has remained largely unknown. In-situ monospecific linear clusters of trilobite arthropods from the lower Ordovician (ca 480 Ma) of Morocco are interpreted here as resulting either from a collective behaviour triggered by hydrodynamic cues in which mechanical stimulation detected by motion and touch sensors may have played a major role, or from a possible seasonal reproduction behaviour leading to the migration of sexually mature conspecifics to spawning grounds, possibly driven by chemical attraction (e.g. pheromones). This study confirms that collective behaviour has a very ancient origin and probably developed throughout the Cambrian-Ordovician interval, at the same time as the first animal radiation events.

No 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 21-20.5 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 et al., 1977; Edel et Lortscher, 1977; Edel, 1979, 1980]. Since then, the end of the rotation fixed at 19 Ma by Montigny et al. [1981] was the subject of discussions and several studies associating paleomagnetism and radiometric dating were undertaken [Assorgia et al., 1994; Vigliotti et Langenheim, 1995; Deino et al., 1997; Gattacceca et Deino, 1999]. This is a contribution to this debate that is hampered by the important secular variation recorded in the volcanics. The only way to get out 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 S12 according to Coulon et al. [1974] or LBLS according to Assorgia et al. [1997] (fig. 1). 40 Ar/ 39 Ar dating of biotites from the debris flow (MDF) has yielded an age of 18.35+ or -0.03 Ma [Dubois, 2000]. Five of the investigated sites are located beneath the debris flow (TV, TVB, TVD, SPM85, SPM86), one site was cored 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 degrees C and one with a reversed polarity and maximum unblocking temperatures at 580-600 degrees C of magnetite. After calculation of a mean direction for each flow, the mean "A1" 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 flow 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 ("C": 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 "A1" direction, the mean "B" 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 upper-most ignimbritic layer of Anglona also emplaced around 18.3 Ma [Odin et 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 "D" 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.