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Antibes, France

Saillet E.,Geosciences Azur | Wibberley C.A.J.,Total S.A.
Journal of Structural Geology | Year: 2010

Cataclastic deformation structures in Cretaceous high-porosity sands in the Bassin du Sud-Est, SE France were surveyed by scan-lines to examine: (i) the role of tectonic loading path on cataclastic deformation band (CDB) network development, and (ii) the development of larger ultracataclastic faults as strain increases. Deformation during Pyrenean-Provençal shortening resulted in a persistent high density (∼10/m2) of conjugate reverse-sense CDB zones (displacements up to ∼30cm), with no generation of larger faults. High-low-density undulations occur for each pair of the conjugate set in an alternating manner, suggestive of network hardening, with a wavelength of several tens of metres being in the order of mechanical bed thickness. For two study areas which experienced significant Oligocene-Miocene extension, a moderate, undulating background density (∼4/m2) of normal-offset CDBs was recorded, which became focussed in places into clusters (∼50/m2) a few metres wide. Thus tectonic loading path may strongly influence strain distribution. CDB zones develop by the addition of successive bands at the edges until, at a thickness of around 5cm, new bands tend to stray further away from the zone edges. Coarser sands have thicker CDB zones, suggesting that host grain size, along with mechanical bed thickness, could be an important contributor to the scale limit in CDB zone growth. Larger ultracataclastic faults and discrete slip zones localised within or at the edges of some clusters of CDB zones, post-date cluster development rather than inducing it. This stage of deformation evolution is only reached in extension, not in shortening, suggesting the infeasibility of achieving the critical state line during horizontal compression. © 2010 Elsevier Ltd. Source

Giresse P.,University of Perpignan | Loncke L.,University of Perpignan | Huguen C.,Total S.A. | Muller C.,6 bis | Mascle J.,Geosciences Azur
Sedimentary Geology | Year: 2010

During the last 10. years several marine geological/geophysical surveys conducted on the Nile deep-sea fan have provided much new information about the large fluid seepage phenomenon, which characterizes this passive margin segment. Among the data, more than 60 sediment cores have been recovered in this area. Rock clasts were collected from eight of them as well as during deep-sea dives, notably in the area of Isis, Osiris and Amon mud volcanoes. In few areas, especially in the Western and the North-eastern provinces of the Nile fan, some clasts consist of crust fragments resulting from cementation of muddy sediment by precipitation of high-Mg calcite. These clasts correspond to a simple cementation of the mud by precipitation of high-Mg calcite: as a consequence no obvious textural or faunal difference with the ambient matrix mud can be observed. These clasts are made of autochthonous crusts derived from microbial anaerobic oxidation of methane and release of bicarbonate and sulphide in the surrounding pore water. These clasts also lack aragonite. In most of the other cases (Menes caldera, Isis, Osiris, and Amon mud volcanoes), allochthonous clasts characterized by low-Mg calcite cement, siderite or dolomite mixed with expelled mud outcrop on the seafloor. Nannofossils have allowed us to date some of these clasts. Most of the ages range from Pliocene to Pleistocene; one clast is Lower Cretaceous in age. The lithological facies often indicate coastal or deltaic environments. Some clasts are closely linked to an evaporite-rich environment (halite, gypsum, and analcime); they all provide useful information on the nature of buried sedimentary series covering the margin. Their presence on the seafloor is due to the expulsion of a pressurized mixture of sediment, water, and gas. This process is believed to have used pre-existing tectonic conduits. © 2010 Elsevier B.V. Source

Obone-Zue-Obame E.M.,University of Perpignan | Gaullier V.,University of Perpignan | Sage F.,Geosciences Azur | Maillard A.,Toulouse 1 University Capitole | And 4 more authors.
Bulletin de la Societe Geologique de France | Year: 2011

The Messinian salinity crisis (MSC) is characterized by gigantic erosion that remodels the margins while a thick, essentially evaporitic and detrital, sedimentary sequence forms in the deep basins. Based on recent (MAURESC, 2003) and earlier (MESEA 1, 1990; MAGIRAA, 1996; GEOBREST, 2002) seismic reflection data, this work brings to light the record of the MSC on the Provençal margin, which has until now been rarely explored from this perspective. Beyond its strictly regional interest, this study fits into a larger synthesis of MSC seismic markers in the Mediterranean and Black Sea marine domain [Lofi et at., 2011] and employs the new nomenclature established on this occasion. The results obtained reveal a Messinian detrital body (CU unit) of 625 metres maximum thickness at the foot of the margin, accumulating at the mouths of the principal canyons. Its form, facies and extension assimilate it to clastic fans, fed by subaerial erosion linked to the MSC. The relative geometry of CU and the Messinian units MU and UU deposited in the deep basin give indications to their chronostratigraphic relations. The deposition of the CU unit is posterior to the basal part of the mobile unit consisting of halite (MU), but contemporary to its top. These results agree with the recent scenarii, which propose that the precipitation of MU in the basin began early, during the lowering of the sea level, and ended at a low level during the MSC [Blanc, 2000; Martinet at., 2001; Sage et at., 2005; Ryan, 20091. The UU unit surmounts MU and is subdivided into two sub-units with perceptibly different seismic facies : UU1 at the base and UU2 at the summit. UU1 could correspond to a unit containing more halite and/or more elastic material than UU2. The UUI sub-unit could be partially contemporary to the CU unit. Concerning salt tectonics and its markers, three structural provinces have been evidenced in the sector of study, respectively : an upslope domain in extension (normal faults), an intermediary domain in translation (tabular MU) and a downslope domain in contraction (salt diapirs). These domains are directly linked to the gravity spreading and/or gliding of the brittle sedimentary cover formed by the CU, UU and Plio-Quatenary units and of the mobile level, MU. In the study area, a close relation between the distribution and thickness of CU and salt tectonics has additionally been evidenced at the mouths of the large Messinian canyons, being best expressed where CU is thick. © 2011. Source

Obayashi M.,Japan Agency for Marine - Earth Science and Technology | Yoshimitsu J.,Japan Agency for Marine - Earth Science and Technology | Nolet G.,Geosciences Azur | Fukao Y.,Japan Agency for Marine - Earth Science and Technology | And 4 more authors.
Geophysical Research Letters | Year: 2013

We present a new whole mantle P wave tomographic model GAP-P4. We used two data groups; short-period data of more than 10 million picked-up onset times and long-period data of more than 20 thousand differential travel times measured by waveform cross correlation. Finite frequency kernels were calculated at the corresponding frequency bands for both long- and short-period data. With respect to an earlier model GAP-P2, we find important improvements especially in the transition zone and uppermost lower mantle beneath the South China Sea and the southern Philippine Sea owing to broadband ocean bottom seismometers (BBOBSs) deployed in the western Pacific Ocean where station coverage is poor. This new model is different from a model in which the full data set is interpreted with classical ray theory. BBOBS observations should be more useful to sharpen images of subducted slabs than expected from simple raypath coverage arguments. Key Points We did a global P tomography using travel times including those from seafloors All data were analyzed using finite frequency kernels We revealed a stagnant slab in the MTZ detached from the downgoing Mariana slab ©2013. American Geophysical Union. All Rights Reserved. Source

Parsons T.,U.S. Geological Survey | Segou M.,Geosciences Azur | Sevilgen V.,Seismicity.net | Milner K.,University of Southern California | And 3 more authors.
Geophysical Research Letters | Year: 2014

We calculate stress changes resulting from the M = 6.0 West Napa earthquake on north San Francisco Bay area faults. The earthquake ruptured within a series of long faults that pose significant hazard to the Bay area, and we are thus concerned with potential increases in the probability of a large earthquake through stress transfer. We conduct this exercise as a prospective test because the skill of stress-based aftershock forecasting methodology is inconclusive. We apply three methods: (1) generalized mapping of regional Coulomb stress change, (2) stress changes resolved on Uniform California Earthquake Rupture Forecast faults, and (3) a mapped rate/state aftershock forecast. All calculations were completed within 24 h after the main shock and were made without benefit of known aftershocks, which will be used to evaluative the prospective forecast. All methods suggest that we should expect heightened seismicity on parts of the southern Rodgers Creek, northern Hayward, and Green Valley faults. Key Points The West Napa earthquake changed stress on Bay Area faultsRapid stress-based earthquake forecasts for evaluating methodsForecasts will be evaluated against the actual aftershock patterns ©2014. American Geophysical Union. All Rights Reserved. Source

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