HR: 0800h
AN: T41A-1171    [Abstracts]
TI: Ocean Floor Orogenesis: Polyphase Metamorphism, Mylonitisation And Deformation Mechanisms Of Metabasites Along The Romanche Transform
AU: Schulmann, K
EM: Schulman@illite.u-strasbg.fr
AF: EOST, Centre de Geochimie de la Surface, 1 rue Blessig, Strasbourg, 67084 France
AU: * Honnorez, J
EM: Honnorez@illite.u-strasbg.frhj
AF: EOST, Centre de Geochimie de la Surface, 1 rue Blessig, Strasbourg, 67084 France
AB: We studied mylonites samples from several dredge-haul profiles accross the Romanche Transform Fault (RTF),in the Equatorial Atlantic. From 7000 to 2500 m water-depths mylonites and ultramylonites grade from high amphibolite (7000 - 6000m), to upper greenschist (6000 - 4000 m), to lower greenschist facies conditions. Deformation temperature during the main mylonitic and synschistose isoclinal folding event systematicaly increases with crustal depth. At each crustal depth mylonitisation degree (grain size reduction down < 1 micrometre) is correlated with increase of metamorphic temperatures determined by hornblend-plagioclase thermometry. The deformation mechanisms range from fracturing and dislocation creep of amphiboles and plagioclase accompanied with chemically induced grain boundary migration in protomylonites and mylonites. Randomizing of CPO and mixing of mineral phases indicate that the grain size sensitive flow operates on all mineral phases in ultrafinegrained ultramylonites of any grade. Mineral compositional zonations and presence of microveins in all crustal depths are consistent with channelised fluid infiltration mechanisms. Heat source for this prograde metamorphic event is ascribed to dissection of gabbroid magma chamber by the RTF. The main metamorphic event was followed by lower greenschist facies metamorphism at all crustal levels. This second event is associated with chevron type refolding of main fabric and dissolution creep deformation mechanism during transpressional regime generating the uplift of the RTF North wall. Lastly, low tempearture metamorphic minerals precipitated in open fractures marking the end of the uplift. The regional extent of prograde metamorphism and polyphase character of deformation and metamorphic recrystallization are consistent with definition of orogensis in the continental lithosphere. Therefore, we propose that the RTF represent an oceanic equivalent of such an orogenesis. The major difference with continental mylonitisation is the abondance of seawater-derived fluids which control all of the metamorphic crystallizations and deformations of the RFZ mylonites.
DE: 8100 TECTONOPHYSICS
DE: 8150 Plate boundary--general (3040)
DE: 1236 Rheology of the lithosphere and mantle (8160)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting