Geology and seismic structure of the northern section of the Oman ophiolite

In the north Oman mountains, a continuous ophiolite succession is exposed, from tectonized harzburgites and dunites at the base, through layered gabbros and peridotites, high-level gabbros and plagiogranite, to a dike swarm and pillowed volcanics overlain by pelagic shales. The upper part of this sequence possesses a static metamorphic overprint, which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high-level intrusives. These pervasively altered rocks are underlain by layered gabbros and peridotites, where hydration is restricted to fractures. Compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water-saturated cores from this sequence. The samples were field-oriented and collected from known stratigraphic levels, which allows the construction of velocity-depth profiles and an examination of anisotropy within the ophiolite. Compressional wave velocities measured for the basalt section are quite variable, ranging from 4.5 to 6.0 km/s at in situ pressures. Within the sheeted dike section, Compressional and shear wave velocities vary from 5.4 to 6.3 km/s and from 2.9 to 3.6 km/s, respectively. From the basalt-sediment contact through the dike section, velocities increase rapidly with depth. This gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity. The top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics. The dikes decrease in abundance downward into the high-level intrusives: coarse-grained gabbroic and granitoid rocks, which often contain abundant hornblende, with isotropic textures and lacking compositional layering. Mean compressional and shear wave velocities increase uniformly from 6.7 and 3.6 km/s, respectively, in the highlevel intrusives, to 7.5 and 3.9 km/s, respectively, at the base of the layered sequence. Measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization. Petrofabrics of relic olivine in the Wadi Ragmi region show strong preferred fabrics for the tectonites, and calculated compressional wave velocities for the serpentine-free rocks range from 7.8 to 8.5 km/s, with the fast direction parallel to the direction of spreading inferred from dike orientations, in excellent agreement with observed upper oceanic mantle seismic anisotropy. The crust-mantle boundary, which is defined by the abrupt increase in seismic velocities, coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites.