Annee 2010 : Séminaires Géosciences Marines

le mardi 9 Mars 2010, 11h00 en salle bleue (T14-IPGP)

Do Hydrothermal Systems Control Detachment Fault Development?

par: Andrew McCaig (Leeds University)

Summary: Hydrothermal systems are generally seen as a response to magmatic events in the oceanic lithosphere. Here we suggest that on slow spreading ridges where magma supply is episodic, hydrothermal circulation may actively control tectonic styles and ridge crest morphology.
Large, long-lived black smoker systems on slow spreading ridge are often closely linked with detachment faults, and for the TAG field it has been suggested that a convex-upward fault controlled hydrothermal upflow leading to venting 4.5 km away from the ridge axis [1,2]. The heat source for hydrothermal flow is thought to be gabbroic intrusions into the detachment footwall, 7 kmbsf and not directly beneath the TAG field[1].
Studies of exposed detachment faults suggest two endmember types; hot detachments (e.g. Atlantis Bank, SWIR) characterised by thick, high temperature (800-950 ¡C) gabbroic mylonite zones, and cold detachments characterised by talc-tremolite schists formed at black smoker temperatures and brittle rather than ductile deformation in gabbroic footwalls. Isotopic data suggests high fluid fluxes along cold detachment faults [2] but only limited flow through footwall gabbros. A simple 1-D thermal model shows that gabbros in the footwall of a detachment held at 400 ¡C by hydrothermal fluid will cool rapidly through the brittle-ductile transition (~750 ¡C), preventing the development of extensive mylonites. Hence the presence of vigorous hydrothermal circulation can profoundly affect crustal rheology.
The cold detachment model requires that the fault is already the locus of hydrothermal flow at the time the gabbro body is emplaced immediately beneath it. We suggest that initially, gabbro is intruded at intermediate depth in the hangingwall of a steep, low displacement axial valley fault. As the gabbro solidifies, the hydrothermal system mines down into it via a cracking front. The next gabbro is intruded into wet rocks and cannot rise so far. Eventually gabbro is intruded beneath the fault at 7 kmbsf. If magma is trapped below the fault it becomes kinematically favourable to switch to extensional faulting on a long-lived detachment as the main mode of plate separation.
Ê[1] deMartin et al. (2007) Geology 35, 711-714. [2] McCaig et al. (2007) Geology 35, 935-938.

le vendredi 12 février 2010 en salle Bleue

NERIES-ESONET OBS – Marine seismology workshop

le Laboratoire de Géosciences Marines

New trends in Marine Seismology
New Technologies
Land/Marine Seismology
Active/Passive Seismology
Future of Marine Seismology
Lien: http://www.ipgp.fr/~rai/

le jeudi 11 février 2010 en salle Bleue

NERIES-ESONET OBS – Marine seismology workshop

le Laboratoire de Géosciences Marines

New trends in Marine Seismology
New Technologies
Land/Marine Seismology
Active/Passive Seismology
Future of Marine Seismology
Lien: http://www.ipgp.fr/~rai/

le mardi 26 Janvier 2010, 11h00 en salle bleue (T14-IPGP)

Subduction mega-earthquakes and other geohazards: IODP NanTroSEIZE as a type example for complex scientific drilling

par: Achim Kopf (Bremen University)

Summary: Subduction zones account for 90% of global seismic moment release, generating damaging earthquakes and tsunamis, with potentially disastrous effects on heavily populated coastal areas. Understanding the processes that govern the strength of earthquakes, and nature and distribution of slip along these plate boundary fault systems, are crucial steps toward evaluating and mitigating geohazards, including tsunamis. As a consequence, the foremost goal of the IODP project NanTroSEIZE is to understand the mechanics and dynamics of seismogenesis and rupture propagation along the active plate boundary faults of a subduction zone, in terms of direct in situ sampling and instrumentation at depth.
NanTroSEIZE is a multi-expedition, multi-platform complex drilling project which eventually will complete a transect of holes the deepest of which will penetrate the seismogenic zone off the Kii Peninsula, Japan, in ca. 6 km depth. Stage 1 drilling included three coordinated riserless expeditions with RV Chikyu to drill several sites across the continental slope and rise in fall 2007 through early 2008. The first of these was a logging while drilling (LWD) expedition that is serving as a geophysical baseline for all of the Stage 1A drilling sites (Expedition 314: LWD Transect). This was followed by a coring expedition (Expedition 315: Megasplay Riser Pilot) aimed at sampling the materials and characterising in situ conditions within the accretionary wedge to 1 km below seafloor at Site C0001 above the Stage 2 drill hole across the deep Òmega-splayÓ out-of-sequence thrust. Expedition 316 (Shallow Megasplay and Frontal Thrusts) targeted another shallow fault zone of the Òmega-splayÓ system in the older accretionary prism (Site C0004) as well as the frontal thrust at the toe of the young accretionary prism (Sites C0006 and C0007).
Initial results from Stage 1A drilling reveal new insights into the stress history and temporal evolution of the Nankai forearc. First, there is no discontinuity in the depositional record between thick forearc basin sediments and the underlying, late Miocene accreted strata (C0002). Second, both borehole breakouts (LWD results) and the orientation of structural measurements on cores suggest a pattern of compression parallel to plate convergence in the wedge, trench-parallel extension above the branches of the mega-splay fault (in particular Site C0001), overlain by trench-orthogonal normal faults in the forearc and below, suggesting predominantly extensional stresses in the overriding accretionary system. Third, the fault zones are highly active given immense problems in borehole stability and core recovery at Sites C0001, C0004, and C0006. The initial Stage 1 shipboard data serve to put forward preliminary hypothesis on the displacement history along the mega-splay and frontal thrust faults. Those data are currently complemented by Stage 2 drilling into the forearc basin, accretionary wedge, and incoming sedimentary sequence as well as installation of borehole observatories(exp 319, 322).

le mardi 19 janvier 2010 à 14 h en salle verte

"OBS Data and Microseismic Activity at Lucky Strike, Mid-Atlantic Ridge"

par Abhishek Rai (Equipe de géosciences marines)

Résumé :
OBS data has been acquired by five Ocean-bottom seismometers, deployed at Lucky-strike slow spreading ridge (37deg. N), Mid-Atlantic. Almost 2 years of continuous record has numerous micro-earthquakes, swarms and non-seismogenic signals. I will present an overview on our observations and seismic activity in the region.

(5 événements)