HR: 10:20h
AN: T12A-01    [Abstracts]
TI: Segmentation in Oman Ophiolite and Fast Spreading Ridges Tectonics
AU: * Nicolas, A J
EM: Adolphe.Nicolas@dstu.univ-montp2.fr
AF: University Montpellier II, Pkace Eugene Bataillon, Montpellier, 34095 France
AU: Boudier, F I
EM: Boudier@dstu.univ-montp2.fr
AF: University Montpellier II, Pkace Eugene Bataillon, Montpellier, 34095 France
AB: New, fine scale mapping in the large NW-SE ridge segment formerly identified in the Oman ophiolite (Nicolas and Boudier, 1995) has revealed that this structure is composed of smaller nestled segments, each being centered on the small mantle diapirs already mapped. The contacts with adjacent lithosphere and the tips of these segments have been mapped in detail. Their nature and structure depend on the difference in age between the two lithospheres. When the difference in age is in the range of 1 Myr, strike slip shear zones, 1-2 km wide, are developed in the mantle of the new segment. When this difference drops to ~0.5 Myr, the shear zones are small and diffuse but, in the mantle wedge at the tip of the segment which penetrates the older lithosphere, spectacular deformations are observed. The mantle and lower crust of the older lithosphere near the Moho are shoveled vertically and kilometer-sized folds develop in the gabbro unit. In contrast, the lid is not affected, suggesting that, at present day fast spreading ridges, similar major tectonic structures, seen in Oman thanks to deep sections, may also be present. Contacts and tips of new segments are invaded by mafic dikes and sills issued from the segment magmatic activity and trapped against these colder boundaries. An important contribution to this magmatism results from massive seawater penetration down to the Moho, possibly favored by the segment tectonic activity. Inside crystallizing magma chamber, the hydrous reaction (Koepke et al.,2004) generates orthopyroxene gabbros which are interlayered with the olivine gabbros. Outside the magma chamber, it generates, by hydrous anatexis, copious melts which mix and react with the indigenous segment melts and crystallize as pargasitic clinopyroxene gabbros and plagiogranites. It is suggested that their magmatic signature should be looked for in present day ridges. Nicolas, A. and Boudier, F., 1995, J.G.R., 100, 6179-6197. Koepke, J., Feig, S.T., Snow, J., Freise, M., 2004. Contrib. Mineral. Petrol. 146, 414-432
DE: 7220 Oceanic crust
DE: 8150 Plate boundary--general (3040)
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting