and the bottom

Recent models of magma chambers at fast-spreading ridges are based on the idea that the entire gabbro section of the oceanic crust crystallizes from a thin melt lens located just below the sheeted dike complex. The shape of the lens has been deduced from seismic reflection data at fast-spreading ridges. On the basis of structural studies in the Oman ophiolite, we suggest that the accretion of the lower crust may not proceed entirely in this way. We emphasize the contrast between: (1) upper level gabbros characterized by a magmatic foliation which, from a flat attitude at depth, rapidly steepen upward and tend to become oriented parallel to the sheeted dikes; and (2) lower gabbros, flat-lying, magmatically deformed, and more or less strongly layered. Wehrlite layers and lenses which contribute to the layering of these gabbros have previously been interpreted as sills. We suggest here that the modally graded bedding, which is an important feature of the lower layered gabbros, may have similarly originated as sills. This is deduced from the fact that, above mantle diapirs, the several hundred metre thickness of the transition zone contains sills of layered gabbros, commonly organized in modally graded sequences. These sills, which are interlayered with dunite or harzburgite, contain gabbros which are shown here to be structurally similar to those in the layered gabbro unit at all scales. If this interpretation is correct, the gabbro section of the oceanic crust in Oman is built up by crystallization, both along the walls and the floor of the perched magma lens, followed by subsidence, and also in sills intruded either in the subsiding foliated gabbros or in the mantle dunites of the Moho transitio