[1] We present new geodetic results of crustal velocities over a large part of northern Asia based on GPS measurements in the Baikal rift zone and Mongolia spanning the 1994-2002 period. We combine our results with the GPS velocity field for China of Wang et al. [2001] and derive a consistent velocity field for most of Asia. We find contrasted kinematic and strain regimes in Mongolia, with northward velocities and N-S shortening in westernmost Mongolia but eastward to southeastward motion and left-lateral shear for central and eastern Mongolia. This eastward to southeastward motion of central and eastern Mongolia is accommodated by left-lateral slip on the E-W trending Tunka, Bolnay, and Gobi Altay faults (2 ± 1.2 mm yr À1 , 2.6 ± 1.0 mm yr À1 , and 1.2 mm yr À1 , respectively) and by about 4 mm yr À1 of extension across the Baikal rift zone. Consequently, $15% of the India-Eurasia convergence is accommodated north of the Tien Shan, by N-S shortening combined with dextral shear in the Mongolian Altay and by eastward displacements along major left-lateral strike-slip faults in central and eastern Mongolia. We find a counterclockwise rotation of north and south China as a quasi-rigid block around a pole north of the Stanovoy belt, which rules out the existence of an Amurian plate as previously defined and implies <2 mm yr À1 of left-lateral slip on the Qinling Shan fault zone.

[1] The Ligurian basin, western Mediterranean Sea, has opened from late Oligocene to early Miocene times, behind the Apulian subduction zone and partly within the western Alpine belt. We analyze the deep structures of the basin and its conjugate margins in order to describe the tectonic styles of opening and to investigate the possible contributions of forces responsible for the basin formation, especially the pulling force induced by the retreating subduction hinge and the gravitational body force from the Alpine wedge. To undertake this analysis, we combine new multichannel seismic reflection data (Malis cruise, 1995) with other geophysical data (previous multichannel and monochannel seismic sections, magnetic anomalies) and constrain them by geological sampling from two recent cruises (dredges from Marco cruise, 1995, and submersible dives from Cylice cruise, 1997). From an analysis of basement morphology and seismic facies, we refine the extent of the different domains in the Ligurian Sea: (1) the continental thinned margins, with strong changes in width and structure along strike and on both sides of the ocean; (2) the transitional domain to the basin; and (3) a narrow, atypical oceanic domain. Margin structures are characterized by few tilted blocks along the narrow margins, where inherited structures seem to control synrift sedimentation and margin segmentation. On the NW Corsican margin, extension is distributed over more than 120 km, including offshore Alpine Corsica, and several oceanward faults sole on a relatively flat reflector. We interpret them as previous Alpine thrusts reactivated during rifting as normal faults soling on a normal ductile shear zone. Using correlations between magnetic data, seismic facies, and sampling, we propose a new map of the distribution of magmatism. The oceanic domain depicts narrow, isolated magnetic anomalies and is interpreted as tholeitic volcanics settled within an unroofed upper mantle, whereas calcalkaline volcanism appears to be discontinuous but massive and has jumped in space and time, from the beginning of rifting on the Ligurian margin ($30 Ma), toward the Corsican margin at the end of the Corsica-Sardinia block rotation ($16 Ma). This space and time shift reveals the importance of the rollback of the Apulian slab and of the migration of the Alpine-Apennines belt front toward the E-SE for driving basin formation. We also state that initial rheological conditions and inherited crustal fabric induce important changes in the styles of deformation observed along margins and between conjugate margins. In the NE Ligurian basin the prerift Alpine crustal thickening together with slow rollback velocity likely contribute to distribute strain across the whole NW Corsican margin, whereas farther south the inherited Hercynian structural pattern combined with a faster rollback of the subducting plate tend to focus the extension at the foot of the margin, up to the Sardinian rift which ends within the SW Corsican margin. Therefore the mode of opening and the margin struct...

[1] We investigate the active seismogenic fault system in the area of the 2003 Mw 6.9 Boumerdes earthquake, Algeria, from a high-resolution swath bathymetry and seismic survey. A series of 5 main fault-propagation folds $20-35 km long leave prominent cumulative escarpments on the steep slope and in the deep basin. Fault activity creates Plio-Quaternary growth strata within uplifted areas such as a rollover basin on the slope and piggyback basins in the deep ocean. Most thrusts turn to fault-propagation folds at the sub-surface and depict ramp-flat trajectories. We find that the two main slip patches of the 2003 Mw 6.9 Boumerdes earthquake are spatially correlated to two segmented cumulative scarps recognized on the slope and at the foot of the margin. The overall geometry indicates the predominance of back thrusts implying underthrusting of the Neogene oceanic crust. Citation: Déverchère, J., et al.

International audienceContinental rifts begin and develop through repeated episodes of faulting and magmatism, but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented. In highly evolved rifts, tensile stresses from far-field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions1, 2, 3. These rifting crises are rarely observed in thick lithosphere during the initial stages of rifting. Here we show that most of the strain during the July–August 2007 seismic crisis in the weakly extended Natron rift, Tanzania, was released aseismically. Deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping. This event provides compelling evidence for strain accommodation by magma intrusion, in addition to slip along normal faults, during the initial stages of continental rifting and before significant crustal thinning

The Messinian salinity crisis (MSC) [Hsü et al., 1973] has deeply shaped the Mediterranean landscape and triggered large sedimentary deposits (evaporites and clastics) in the deep basins within a short time span. Until recently, the MSC has mainly been analyzed independently, either through outcrops located onshore (e.g. Morocco, Cyprus, Spain, Sardinia, Italy) or through marine seismic profiles in the deep offshore. Each approach bears its own limitations:(1) on the one hand, land outcrops refer to incomplete Messinian successions that are geometrically disconnected from the offshore Messinian deposits owing to tectonics (e.g. Apennines) and/or because they accumulated at an early stage of the crisis in shallow marginal basins (e.g. Spain); (2) on the other hand, seismic profiles from the upper margins down to the deep basins allow to image and explore the entire MSC event as a continuous process, but with a lower resolution and with a lack of stratigraphical and lithological control, in the absence of full recovery of scientific boreholes.We present here a synthesis of a set of modern geophysical data over the Mediterranean and Black seas allowing to image the Messinian markers (erosion surfaces, depositional units and their bounding surfaces) much better than previously and to study the spatio-temporal organisation of these markers from the inner-shelves down to the bathyal plains. The results from thirteen areas located offshore are compared, with common charts and nomenclatures. The comparative and multi-site approach developed here allows to analyse the record of the MSC on margin segments and basins that depict various structural, geodynamical and geological settings, to fix a number of local influencing factors (tectonics, subsidence, inherited topography, sedimentary fluxes...) and to partly assess their influence in facies and geometrical variations of the MSC units. We are thus able to extract from our analysis some recurrent signals related to the MSC ss., allowing us to discuss: (1) the amplitude and modalities of base-level changes during the MSC; (2) the depositional modalities of the MSC units in the deep basins; (3) the location of the erosion product of the margins and to emphasise (4) the major differences between the eastern and western Mediterranean basins. Une meilleure connaissance des enregistrements de la crise de salinité messinienne en domaine marin grâce à l'analyse sismique multi-sitesMots-clés. -Crise de salinité messinienne, Méditerranée, Profils sismiques, Evaporites, Erosion, Clastiques.Résumé. -La crise de salinité messinienne (CSM) [Hsü et al., 1973] a profondément modelé les paysages méditerra-néens et généré d'épaisses accumulations sédimentaires (évaporites et dépôts clastiques) dans les bassins profonds sur une brève période de temps à l'échelle géologique. Jusqu'à présent, la CSM a principalement été étudiée distinctement, à terre, grâce aux affleurements (ex. Maroc, Chypre, Espagne, Sardaigne, Italie…), et en domaine marin, par l'intermé-diaire de profils sismiques. Chacun...

We present new results from the MARADJA'03 cruise depicting the geological structures offshore central and western Algeria. Using swath bathymetry and seismic reflection data, we map and discuss the offshore limits of the Internal Zones corresponding to relics of the AlKaPeCa domain that drifted and collided the African plate during the Miocene. We identify large reverse faults and folds that reactivate part of these limits and are still active today. The morphology of the westernmost NE-SW margin suggests a former strike-slip activity accommodating a westward block translation responsible for the shift of the Internal Zones towards the Moroccan Rif.Résumé: Nous présentons les résultats récents de la campagne MARADJA'03, qui visent à mettre en évidence les structures géologiques dans le domaine marin au nord-ouest de l'Algérie. Grâce aux données de bathymétrie multifaisceau et de sismique réflexion, nous cartographions et discutons les limites en mer des Zones internes correspondant aux reliques du domaine AlKaPeCa qui a dérivé, puis est entré en collision avec la plaque africaine au Miocène. De grandes failles inverses et plis, actifs dans le champ de contrainte actuel, réactivent certaines de ces limites. La marge ouestalgérienne, orientée NE¿SW, indique la présence d'une ancienne activité en décrochement ayant accommodé la translation des Zones internes vers l'ouest.

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