Résumé
With its coherent lithological sequence covering the upper basalts, sheeted dikes, gabbros and the upper mantle, the Oman Ophiolite provides the best analog for fast-spreading oceanic crust on land. Wadi Gideah in the Wadi Tayin Block covers the entire sequence, and was therefore chosen for establishing a reference profile for multiple method analysis. The spatial resolution of microstructural data from the surface reference profile (average spacing approximately 80 m) was significantly refined by two 400 m long drill cores obtained in the frame of the ICDP Oman Drilling Project. These cores cover the foliated/layered gabbro transition (GT2) and the lower layered gabbros (GT1), and were sampled with an average spacing of 6.5 and 8.5 m, respectively. We quantified structural features using the ODF J index (from 1 for a random fabric to infinity for a single crystal) and the BA index (from 0 for purely foliated fabric to 1 for a purely lineated fabric) taking the pole figures of every sample into account. Crystallographic preferred orientations (CPO) of plagioclase and clinopyroxene from the surface profile show that in the upper 1500 m of the lower crust the plagioclase and clinopyroxene fabrics are both foliated and lineated. At 3500 m above the mantle/crust boundary, the fabrics change towards almost purely foliated, and then progressively more lineated down section. This trend is accompanied by scattered CPO strength with significantly changing pole figure symmetries along the lowermost 3500 m of the crust. Substantial microstructural variation in fabric strength and symmetry also on the decimeter scale was observed in the drill cores confirming that the accretion mechanism of the upper 1500 m differs from that of the lower 3500 m. We interpret the lineation in the upper part as result of a downward magmatic flow whereas the variability along the lower section is consistent with in situ crystallization and deformation of individual melt reservoirs. The CPO strength increasing down section with gradually increasing lineation in the lower 3500 m agrees with active mantle flow shearing part of the lower crust during its solidification. Very low degrees of internal misorientation indicate that crystal-plastic deformation did not play a major role and can be explained by low amounts of residual melt accommodating minor plastic deformation.