VARIABLE CRUSTAL THICKNESS IN THE OMAN OPHIOLITE - IMPLICATION FOR OCEANIC-CRUST

From 32 detailed cross sections through the gabbro unit of the Oman op hiolite, it is concluded that the thickness of this unit is on average 3.6 km. The lower layered gabbros represent two thirds, and the upper homogeneous foliated gabbros represent one third the gabbro unit. Ass uming that the overlying basaltic lid (sheeted dikes and extrusives) i s 1.5-2 km thick, the average crustal section in the Oman ophiolite is 0.5-1 km thinner than the standard 6-km-thick oceanic crust usually c onsidered to be produced at fast spreading ridges, a point which is di scussed. Variations in gabbro thickness between 5.4 km and 1.5 km are recorded. There is a general correlation throughout the ophiolite belt , particularly in the southeastern massifs, such that the thinnest gab bro units (2.2-2.5 km thick) overlay the thickest (300-700 m) transiti on zones which separate them from the mantle harzburgites and the thic kest gabbro units (3.6-3.9 km thick) overlay the thinnest (5-100 m) tr ansition zones. Tile combination of thinner gabbro units and thicker t ransition zones is observed above mantle diapirs or in domains which, following our structural models, were accreted above diapirs and have drifted in the spreading direction. If it is assumed that the extrusiv e basalt and the sheeted dike complex units have a constant thickness, such large variations indicate similar variations in the Moho level b elow the ridge of origin; in particular, the Moho above mantle diapirs should be some 1-1.5 km shallower than away from diapirs. As the Oman ophiolite is considered to derive from a fast spreading paleoridge, t his doming should be detected in actual fast spreading ridges, as sugg ested by Earth and Mutter [this issue] and Wang et al. [this issue].