Geosciences Rennes

Rennes, France

Geosciences Rennes

Rennes, France
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Kydonakis K.,Geosciences Rennes | Brun J.-P.,Geosciences Rennes | Sokoutis D.,University Utrecht | Sokoutis D.,University of Oslo | Gueydan F.,Montpellier University
Tectonophysics | Year: 2015

In the Chalkidiki Peninsula of northern Greece a thrust complex made of a basement (Vertiskos Unit), a cover (Circum-Rhodope belt) and arc/back-arc units (Chortiatis Magmatic Suite and eastern Vardar Ophiolites) is exposed in the Chalkidiki Peninsula of northern Greece. The complex forms the western part of the Rhodope Metamorphic Province and lies on the hanging-wall of the Kerdylion Detachment, the structure responsible for the exhumation of the Southern Rhodope Core Complex and the most prominent and visible ductile structure related to the Tertiary Aegean extension. The Chalkidiki thrust complex arguably preserves a complete record of Cretaceous deformation and related fabrics. In this contribution we describe the geometry of foliation, stretching lineation and shear sense(s) on a regional scale. The regional foliation strikes NW-SE and displays different patterns in the three studied units: (i) dominantly dipping at low angle in the Vertiskos Unit, (ii) affected by upright folding in the Circum-Rhodope belt and (iii) systematically steeply dipping to the NE in the Chortiatis Magmatic Suite. Stretching lineation trend dominantly SW-NE in the three mentioned units. On the basis of new mapping, neglecting local perturbations and deformation related to Tertiary extension, we infer the regional kinematics of Cretaceous syn-metamorphic thrusting and subsequent exhumation of the metamorphic units. Thrusting took place toward the SW (in present-day coordination) and the related fabrics are recorded throughout the metamorphic pile. On the contrary, exhumation-related fabrics are related to shear toward the NE and are preferentially recorded in the uppermost part of the metamorphic pile suggesting that extension was more localised and of less finite intensity compared to thrusting. © 2015 Elsevier B.V.

Blaich O.A.,University of Oslo | Faleide J.I.,University of Oslo | Tsikalas F.,University of Oslo | Tsikalas F.,ENI S.p.A | And 4 more authors.
Petroleum Geology Conference Proceedings | Year: 2010

Regional seismic reflection profiles and potential field data across the conjugate magma-poor Camamu/Almada-Gabon margins, complemented by crustal-scale gravity modelling and plate reconstructions, are used to reveal and illustrate the relationship of crustal structure to along-margin variation of potential field anomalies, to refine and constrain the continent-ocean boundary, as well as to study the structural architecture and nature of the continent-ocean transitional domain. The analysis reveals that the prominent conjugate Salvador-N'Komi transfer system appears to be a first-order structural element, governing the margin segmentation and evolution, and may have acted as an intraplate decoupling zone. The continent-ocean transitional domain, offshore northeastern Brazil, is characterized by rotated fault-blocks and wedge-shaped syn-rift sedimentary sequences overlying a prominent and undulated reflector ('M-reflector'), which in turn characterizes the boundary between an extremely thinned, possibly magmatically intruded, continental crust and normal lithospheric mantle. The 'M-reflector' in the northeastern Brazilian margin shows remarkable similarities to the S-reflector at the West Iberia margin. In the same way, the 'M-reflector' is interpreted as a detachment surface that was active during rifting. Unlike the well studied central and northern segments of the West Iberia margin, however, the present study of the northeastern Brazilian margin does not clearly reveal evidence of an exhumation phase. The latter predicts exhumation of middle and lower crust followed by mantle exhumation. Increase in volcanic activity during the late stages of rifting may have 'interrupted' the extensional system, implying a failed exhumation phase. In this setting, the break-up and drift phase may have replaced the exhumation phase. Nevertheless, the available observations cannot discount the possibility that the 'M-reflector' is underlain by partially serpentinized mantle. Our study further leads to the development of a detailed conceptual model, accounting for the complex tectonomagmatic evolution of the conjugate northeastern Brazilian-Gabon margins. This model substantiates a polyphase rifting evolution mode, which is associated with a complex time-dependent thermal structure of the lithosphere. In the conjugate margin setting, asymmetrical lithospheric extension resulted in the formation of the thinned continental crust domain prior to the formation of the approximately symmetrical transitional domain. © Petroleum Geology Conferences Ltd. Published by the Geological Society, London.

Rubino J.L.,Total S.A. | Pendkar N.,Petronas | Baby G.,University of Maputo | Blanpied C.,Total S.A. | And 4 more authors.
1st EAGE Eastern Africa Petroleum Geoscience Forum - Sharing Lessons Learned: What's Next? | Year: 2015

New investigations carried out onshore Nacala in Mozambique prove the occurrence of Oxfordian to Kimmeridgian marine series. This series overlie an older series (Jurassic?) with a marked angular unconformity. These two units demonstrate the occurrence of a proto Mozambique marine channel West of the Davies Ridge and are in line with the results of the offshore exploration wells and help to improve the seismic interpretation at depth.

Leroy S.,University Pierre and Marie Curie | Razin P.,Bordeaux Montaigne University | Autin J.,University Pierre and Marie Curie | Autin J.,French National Center for Scientific Research | And 29 more authors.
Arabian Journal of Geosciences | Year: 2012

We present here a synthesis of the evolution of rifted continental margin systems in the Gulf of Aden. These margins are volcanic to the west of the Gulf of Aden, where they are influenced by the Afar hotspot, and nonvolcanic east of longitude 46° E. The combined use of magnetics, gravity, seismic reflection, field observations (tectonic, stratigraphic and sedimentological) and oil well data allowed us to obtain better constraints on the timing of continental rifting and seafloor spreading. From the Permo-Triassic to the Oligocene, the Arabian-African plate was subject to distributed extension, probably due, at least from the Cretaceous, to tensile stresses related to the subduction of the Tethysian slab in the north. In Late Eocene-Early Oligocene, 34-33 Ma ago, rifting started to localise along the future area of continental breakup. Initially guided by the inherited basins, continental rifting then occurred synchronously over the entire gulf before becoming localised on the northern and southern borders of the inherited grabens, in the direction of the Afar hotspot. In the areas with non-volcanic margins (in the east), the faults marking the end of rifting trend parallel to the inherited grabens. Only the transfer faults cross-cut the inherited grabens, and some of these faults later developed into transform faults. The most important of these transform faults follow a Precambrian trend. Volcanic margins were formed in the west of the Gulf, up to the Guban graben in the southeast and as far as the southern boundary of the Bahlaf graben in the northeast. Seaward dipping reflectors can be observed on many oil industry seismic profiles. The influence of the hotspot during rifting was concentrated on the western part of the gulf. Therefore, it seems that the western domain was uplifted and eroded at the onset of rifting, while the eastern domain was characterised by more continuous sedimentation. The phase of distributed deformation was followed by a phase of strain localisation during the final rifting stage, just before formation of the Ocean-Continent Transition (OCT), in the most distal graben (DIM graben). About 20 Ma ago, at the time of the continental break-up, the emplacement of the OCT started in the east with exhumation of the subcontinental mantle. Farther west, the system was heated up by the strong influence of the Afar hotspot, which led to breakup with much less extension. In the Gulf of Aden (s.str), up to the Shukra El Sheik fracture zone, oceanic spreading started 17.6 Ma ago. West of this fracture zone, oceanic accretion started 10Ma ago, and 2Ma ago in the Gulf of Tadjoura. Post-rift deformation of the easternmargins of the Gulf of Aden can be seen in the distal and proximal domains. Indeed, the substantial post-rift uplift of these margins could be associated with either the continental breakup, or activity of the Afar hotspot and related volcanic/magmatic activity. Uplift of the northern proximal margin was still active (e.g. stepped beach rocks exposed at 60 m of 2 Ma; 30mof 35,200 years; 10 and 2m) and active volcanoes can be inferred at depths of between 70 and 200 km beneath the margin (at 5-10 km distance from the coast). On the distal margin, heat flow measurements show a high value that is associated with post-rift volcanic activity and the development of a volcano (with flows and sills) shortly after the formation of the OCT. The Afar hotspot is therefore important for several reasons. It allows the localisation of deformation along the Red Sea/Aden system and the rapid opening of the Gulf after the continental break-up; its influence also seems to persist during the post-rift period. © Saudi Society for Geosciences 2011.

Le Guerroue E.,Geosciences Rennes | Cozzi A.,Eni Angola
Gondwana Research | Year: 2010

There is widespread interest in the Neoproterozoic period of the Earth's history (1000 to 542 Ma) because of unprecedented δ13C fluctuations to < - 10‰ PDB through thick (> 1000 m) succession of stratigraphically complex sedimentary rocks deposited during tens of millions of years. In contrast, Phanerozoic large negative C-isotope excursions have been interpreted as the result of diagenetic fluid mixing during carbonate stabilization and burial and are less enigmatic due to the excellent biostratigraphic control on their timing and duration. The Ediacaran Nafun Group of Oman (part of the Huqf Supergroup spanning the Cryogenian-Early Cambrian) contains a large δ13C negative excursion (the Shuram excursion) reaching values as negative as - 12‰ at the base of the Shuram Formation. A steady recovery to positive values occurs over the entire Shuram and half through the overlying Buah Formation, suggesting a duration on the order of tens of My. Based on trace metal, chemostratigraphic and sedimentological analyses, the carbon isotope record obtained from the Buah Formation of northern Oman indicates a systematic and reproducible shift of δ13C values from - 6‰ to + 1‰ in 1 - a demonstrably diagenetic altered carbonate-cemented siliciclastic facies, and 2 - a least diagenetically altered stromatolitic facies. The identical reproducible isotopic pattern in these time-equivalent sections combined to the presence of exceptionally preserved δ18O values around - 2 to + 1‰ associated with the most negative δ13C values rules out isotopic resetting by diagenetic fluids as a mechanism to explain these values. It is concluded that it is possible to retain depositional δ13C values in demonstrably diagenetically altered carbonates. This raises the issue of the ability to recognize diagenetic alteration of C-isotopic values in Neoproterozoic rocks where a robust time frame to support reproducibility is not available. The results of this study provide strong support to a non diagenetic origin of the negative Shuram C-isotope excursion, believed to be the most profound (in terms of amplitude and duration) in the Earth's history. © 2009 International Association for Gondwana Research.

Manzotti P.,University of Bern | Ballevre M.,Geosciences Rennes | Zucali M.,University of Milan | Zucali M.,CNR Institute for the Dynamics of Environmental Processes | And 2 more authors.
Swiss Journal of Geosciences | Year: 2014

This study reviews and synthesizes the present knowledge on the Sesia–Dent Blanche nappes, the highest tectonic elements in the Western Alps (Switzerland and Italy), which comprise pieces of pre-Alpine basement and Mesozoic cover. All of the available data are integrated in a crustal-scale kinematic model with the aim to reconstruct the Alpine tectono-metamorphic evolution of the Sesia–Dent Blanche nappes. Although major uncertainties remain in the pre-Alpine geometry, the basement and cover sequences of the Sesia–Dent Blanche nappes are seen as part of a thinned continental crust derived from the Adriatic margin. The earliest stages of the Alpine evolution are interpreted as recording late Cretaceous subduction of the Adria-derived Sesia–Dent Blanche nappes below the South-Alpine domain. During this subduction, several sheets of crustal material were stacked and separated by shear zones that rework remnants of their Mesozoic cover. The recently described Roisan-Cignana Shear Zone of the Dent Blanche Tectonic System represents such a shear zone, indicating that the Sesia–Dent Blanche nappes represent a stack of several individual nappes. During the subsequent subduction of the Piemonte–Liguria Ocean large-scale folding of the nappe stack (including the Roisan-Cignana Shear Zone) took place under greenschist facies conditions, which indicates partial exhumation of the Dent Blanche Tectonic System. The entrance of the Briançonnais micro-continent within the subduction zone led to a drastic change in the deformation pattern of the Alpine belt, with rapid exhumation of the eclogite-facies ophiolite-bearing units and thrust propagation towards the foreland. Slab breakoff probably was responsible for allowing partial melting in the mantle and Oligocene intrusions into the most internal parts of the Sesia–Dent Blanche nappes. Finally, indentation of the Adriatic plate into the orogenic wedge resulted in the formation of the Vanzone back-fold, which marks the end of the pervasive ductile deformation within the Sesia–Dent Blanche nappes during the earliest Miocene. © 2014, Swiss Geological Society.

Chauvel C.,Joseph Fourier University | Maury R.C.,CNRS Oceanic Domains Laboratory | Blais S.,Geosciences Rennes | Lewin E.,Joseph Fourier University | And 4 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2012

The scale and geometry of chemical and isotopic heterogeneities in the source of plumes have important scientific implications on the nature, composition and origin of plumes and on the dynamics of mantle mixing over time. Here, we address these issues through the study of Marquesas Islands, one of the Archipelagoes in Polynesia. We present new Sr, Nd, Pb, Hf isotopes as well as trace element data on lavas from several Marquesas Islands and demonstrate that this archipelago consists of two adjacent and distinct rows of islands with significantly different isotopic compositions. For the entire 5.5 Ma construction period, the northern islands, hereafter called the Ua Huka group, has had systematically higher 87Sr/ 86Sr and lower 206Pb/ 204Pb ratios than the southern Fatu Hiva group at any given 143Nd/ 144Nd value. The shape and curvature of mixing arrays preclude the ambient depleted MORB mantle as one of the mixing end-members. We believe therefore that the entire isotopic heterogeneity originates in the plume itself. We suggest that the two Marquesas isotopic stripes originate from partial melting of two adjacent filaments contained in small plumes or "plumelets" that came from a large dome structure located deep in the mantle under Polynesia. Low-degree partial melting under Marquesas and other "weak" Polynesian hot spot chains (Pitcairn-Gambier, Austral-Cook, Society) sample small areas of the dome and preserve source heterogeneities. In contrast, more productive hot spots build up large islands such as Big Island in Hawaii or Réunion Island, and the higher degrees of melting blur the isotopic variability of the plume source. © 2012. American Geophysical Union. All Rights Reserved.

Pasquet S.,French National Center for Scientific Research | Bodet L.,French National Center for Scientific Research | Longuevergne L.,Geosciences Rennes | Dhemaied A.,French National Center for Scientific Research | And 3 more authors.
Near Surface Geoscience 2012 | Year: 2012

In the context of a geophysical survey at the Ploemeur hydrological observatory (France), we performed surface-wave profiling for the characterisation of shallow subsurface Shear-wave velocities. Since we anticipated lateral variations but needed great investigation depth, we deployed multifold acquisition geometries and used roll-along dispersion stacking to enable efficient measurements of multi-modal dispersion data. Several offset moving windows have been tested. Represented as pseudo-sections, the phase velocities extracted using a 12-trace window clearly showed three areas coherent with field observation and interestingly consistent with electrical conductivities and P-wave first arrival times. This cross-quality control has been of great help in the choice of the moving window size and revealed itself to be a rewarding step prior to the inversion process.

Rouby D.,Geosciences Environment Toulouse | Chardon D.,Geosciences Environment Toulouse | Guillocheau F.,Geosciences Rennes | Robin C.,Geosciences Rennes
EAGE/AAPG Workshop 2013: Basin-Margin Wedge Exploration Plays | Year: 2013

The thermal and flexural evolution of passive margins are impacted by the (un)loading effects of erosion/sedimentation processes, which, in turn, affect their relief and sediment accumulation. This complex coupling is recorded by the stratigraphic trend of the associated sedimentary basins, which is controlled by the balance between sediment accumulation, subsidence and eustasy.

Brun J.-P.,Geosciences Rennes | Fort X.,G.O. Logical Consulting
Marine and Petroleum Geology | Year: 2011

Salt tectonics at passive margins is currently interpreted as a gravity-driven process but according to two different types of models: i) pure spreading only driven by differential sedimentary loading and ii) dominant gliding primarily due to margin tilt (slope instability). A comparative analysis of pure spreading and pure spreading is made using simple mechanics as well as available laboratory experiments and numerical models that consider salt tectonic processes at the whole basin scale. To be effective, pure spreading driven by sedimentary loading requires large differential overburden thicknesses and therefore significant water depths, high sediment density, low frictional angles of the sediments (high fluid pore pressure) and a seaward free boundary of the salt basin (salt not covered by sediments). Dominant gliding does not require any specific condition to be effective apart from the dip on the upper surface of the salt. It can occur for margin tilt angles lower than 1° for basin widths in the range of 200-600. km and initial sedimentary cover thickness up to 1. km, even in the absence of abnormal fluid pressure. In pure spreading, salt resists and sediments drive whereas in dominant gliding both salt and sediments drive. In pure spreading, extension is located inside the prograding sedimentary wedge and contraction at the tip. Both extension and contraction migrate seaward with the sedimentary progradation. Migration of the deformation can create an extensional inversion of previously contractional structures. In pure spreading, extension is located updip and contraction downdip. Extension migrates downdip and contraction updip. Migration of the deformation leads to a contractional inversion of previously extensional structures (e.g. squeezed diapirs). Mechanical analysis and modelling, either analogue or numerical, and comparison with margin-scale examples, such as the south Atlantic margins or northern Gulf of Mexico, indicate that salt tectonics at passive margins is dominated by dominant gliding down the margin dip. On the contrary, salt tectonics driven only by differential sedimentary loading is a process difficult to reconcile with geological evidence. © 2011 Elsevier Ltd.

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