SEGMENTATION IN OMAN OPHIOLITE, IMPLICATIONS FOR FAST SPREADING RIDGES TECTONICS AND HIGH TEMPERATURE HYDROTHERMAL ACTIVITY

Abstract

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 shovelled 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, as 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. Melt reaction in the uppermost mantle at the tip of propagators generates large volumes of dunites and wehrlites. An important contribution to this magmatism results from massive seawater penetration down to the Moho, possibly favored by the segment tectonic activity. This water is heated at very high temperatures (VHT), 1000C° and possibly above. Inside crystallizing magma chamber, the hydrous reaction (Koepke et al.,2004) rejuvenates magmatic activity and generates orthopyroxene gabbros, injected and stretched parallel to the olivine gabbros layering. Outside the magma chamber, VHT seawater produces, by hydrous anatexis, copious melts which mix and react with the indigenous segment melts and crystallize as orthopyroxene- or pargasitebearing, clinopyroxene gabbros, diorites and trondjhemites. It is suggested that the geochemical signature of such hybrid melts should be looked for in present day ridges.