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Rennes, France

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.

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.

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.

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.

Agency: GTR | Branch: NERC | Program: | Phase: Research Grant | Award Amount: 400.36K | Year: 2008

There are a number of rocky bodies in our Solar System, including our own planet and moon. Largely they have similar internal structures. At the heart is an iron core, surrounded by a solid silicate mantle, which itself is covered by a thin silicate crust. This crust was (and still is) formed when the mantle partially melted and the less dense magmas rose to the surface. The crust of all planets is mostly made of a rock type called basalt. However, the Earth appears to be unique in the Solar System in that the basaltic crust only covers two-thirds of the surface (mostly covered by the worlds oceans). The other one third is made of material that is more silica-rich and significantly thicker than the basaltic crust. These are the continents we live on. Exactly when this peculiar feature of our planet formed is one of the central, longest-lived mysteries in the study of the Earth. Using radioactive elements, the age of crustal rocks can be dated. The age of thousands of samples has been measured, and what is clear is that the continental crust is not all the same age and some parts of it are very, very old. The Earth itself is 4.55 billion years old and some crustal material is as old as 4.4 billion years. In fact, there are four ages of continental crust that appear over and over again: 1.2, 1.9, 2.7 and 3.3 billion years. Some have interpreted this repetition to mean that these were times of accelerated continental growth, pulses of magmatism. However, it may be that the record of ages is largely incomplete, and the paucity of continental crust at other ages may not mean that growth rates were low at those times. It might just be that crust of those ages was destroyed by erosion, a process we can see happening today. The data is fundamentally ambiguous. It is analogous to looking through someones diary and finding missing dates. Did nothing happen on those days? Or were the pages torn out? Our research aims to understand the formation of the continental crust, not by looking at the crust itself, but by looking at the mantle. Radioactive elements in the mantle should have recorded the time when the melts that formed the crust were extracted. If the same pulses of magmatism that are seen in the continents are also found in the mantle, it would confirm the idea of pulsed continental growth. Further, it would suggest that during these events, there was massive magmatism on the planet, far greater than at any time since. If the peaks are not found in the mantle, then it is likely the crustal age peaks were produced primarily by erosion. Our research focuses on the isotope 187Os, because osmium has unique chemical properties that makes it a more robust recorder of melting ages than other isotopes. The study will take advantage of recent advances in analytical technology and most of the analyses will be done by ablating samples with a laser. This will allow a large amount of data to be acquired in a short time. While understanding the formation of the continents is a worthy topic in itself, understanding its growth may have broader implications. A growing set of observations suggests that the Earths atmosphere and oceans have undergone radical changes in the planets ancient past, including the abrupt rise of oxygen in the atmosphere that is essential to all animal life. These changes had major effects on the course of biologic evolution. What caused these changes is not clear. Intriguingly, a number of the atmospheric/evolutionary shifts seem to correspond in age to the apparent crustal growth pulses. If the pulses were times of massive, global magmatism, it is likely they would have had a profound effect on the composition of the atmosphere and oceans, as even single volcanic eruptions have been observed to change the global climate. This raises the interesting possibility that the large-scale pattern of lifes evolution was set by catastrophic events in the Earths interior.

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