Articles

Séminaire GM/LGO d’Océane FOIX (GM, Ifremer, Brest)

The 3 ‐ D Velocity Models and Seismicity Highlight Forearc Deformation

Due to Subjecting Features (Central Vanuatu)

 

The central Vanuatu forearc is characterized by a reduced convergence rate at the trench, signifcant uplif of the overriding plate, and the presence of large forearc islands. Volcanic actvity and intermediate ‐ depth seismicity behind the forearc are among the highest on Earth. These features are presumed to be associated with the subducton of a large seamount chain and an immersed ridge. We used a catalog of P and S arrivals from a local seismological network to construct the frst 3 ‐ D velocity model of the region and to relocate earthquakes beneath the forearc. The 3 ‐ D model reveals a highly heterogeneous velocity distributon in the frst 40 km beneath the surface. Trench ‐ parallel low P and S velocity zones in the upper tens of kilometers beneath the western edges of the two largest forearc islands correlate to the major features entering into subducton and suggest highly fractured and probably water ‐ infltrated features. Trench ‐ parallel high ‐ velocity zones at 5–15 ‐ km depth, further to the east, may be part of a contnuous consolidated rock structure that acts as a backstop. Thick overriding plate crust (29 ± 3 km) in the forearc is consistent with the presence of contnental remnants. The earthquake distributon is generally heterogeneous, suggestng a complex fault structure and variable stress. Earthquakes are, however, well aligned at the plate interface in between the subductng features, where they constrain the angle of subducton to be 15° on average, down to 10–15 ‐ km depth.

6 mars 2020

Séminaire conjoint GM/LGO d’Eric Deville (IFP Énergies nouvelles, Rueil-Malmaison)

H2-rich natural gas seepages on Earth: where do they come from?

 

We present the results of studies (geological context and geochemical characteristics) concerning H 2 -rich gas seepages in different areas of the Earth, comparing offshore and onshore geological contexts, and focusing notably in exhumed terrestrial mantle rocks (ophiolitic rocks) and in intraplate domains (including at depth old continental crust). These gas seepages are in all cases made up of mixtures of 3 main components which are H 2 , CH 4 and N 2 in various proportions, with locally relatively high contents in helium in onshore context. Furthermore, in hydrothermal volcanic systems, the gas mixtures are associated with the presence of CO 2 . The relative contents of the dominant gases show distinct characteristic types which are interpreted as reflecting different zones of gas generation. In all cases, it appears that H 2 generation is mainly favored by the oxidation of Fe II -rich rocks responsible for water reduction (hydrolysis). In absence of CO 2 , H 2 -bearing gases are either associated to ultra-basic springs (pH 10-12) or they seep directly within the fracture systems of the rocks. In volcanic contexts, pH decreases drastically correlated with the presence of acid gases. CH 4 is produced either directly by reaction of dissolved CO 2 with basic-ultrabasic rocks or, in a second step, by H 2 -CO 2 interaction by abiotic or microbial processes. H 2 is present in the gas when no more carbon is available to generate CH 4 (carbon restriction conditions) or when the H 2 -CO 2 association corresponds to an active and dynamic unstable system (case of volcanic gases along mid-oceanic ridges, hot-spot areas and arc systems). N 2 is issued either from water in equilibrium with atmosphere, or, in case of important enrichment in N 2 and He, it is interpreted as issued mainly from the continental crust, respectively by ammonium destabilization within metamorphic clay and radiogenic decay. Experiments have shown that the presence of ammonium favors H 2 generation. This type of study shows that, despite of a common prejudice, H 2 is not so rare in natural environments and this native H 2 can be seen as a potential resource of clean energy commonly associated with helium for which the demand is constantly increasing.

14 février 2020

Séminaire de Luc Bulot (CEREGE / Aix-Marseille Université)

Retour sur le terrain: Une réponse aux limites d’application des échelles stratigraphiques standards à haute résolution

Séminaire GM/LGO de Marco TERZARIOL (Géosciences Marines, Ifremer, Brest)

Methane Hydrate-Bearing Sediments: The Role of Fines

 

Methane hydrate-bearing sediments (MHBS) are relevant to the organic carbon cycle, seafloor instability, and as a potential energy resource. Sediment characteristics affect hydrate formation, gas migration, and potential recovery strategies. The first part of this presentation will focus on testing the Geotech properties of natural hydrate-bearing sediments. Finally, we combine the physics of granular materials with robust physical models to anticipate the pore habit of methane hydrates as a function of the sediment characteristics and depth. Results highlight the critical role of fines on sediments mechanical and flow properties, hydrate pore habit, and potential fluid migration.

Séminaire de Pascal Philippot (Géoscience Montpellier)

L’oxygénation de la Terre: point de non- retour global mais registre local