EFFECTS OF COMPOSITION AND SERPENTINIZATION ON SEISMIC PROPERTIES OF THE OCEANIC LITHOSPHERE

Based on an idealized ophiolite sequence formed at fast-spreading ridges, from top to bottom, the oceanic lithosphere consists of the ocean plate stratigraphy, a layered mafic-ultramafic plutonic complex crystallized from a magma chamber, and ultramafic tectonites deformed by the mantle flow. The lattice preferred orientation of olivine in ductile deformed tectonites indicates dominant activation of [100](010) slip system (the A-type fabric), which results in the fastest P-wave velocity and the polarization direction of the faster S-wave velocity parallel to the mantle flow direction. Therefore the petrologic Moho separates the sheared, anisotropic mantle from the nearly isotropic crust of a magmatic origin. Theoretically, sharpness of the seismic Moho depends on the sharpness of petrologic transition from olivine-rich to olivine-poor compositions in the lowest oceanic crust. However, the Moho in ocean basins is absent in 40% areas. Velocity measurements on peridotite samples from ocean drilling boreholes and ophiolites reveal that widespread serpentinization significantly decreased P- and S-wave velocities of peridotites and reduced the Moho reflectivity, but did not affected the pattern of seismic anisotropy.

Lizardite and chrysotile controls serpentinization of the oceanic lithosphere below 300 ˚C, whereas antigorite is more stable in the forearc upper mantle and subduction zones until 600-650 °C. Poisson’s ratios of lizardite and antigorite are 0.267 and 0.263, respectively, just slightly higher than those of olivine and pyroxene. In contrast, chrysotile and clinochlore show extremely high Poisson’s ratio of 0.359 and 0.336, respectively, which make them favorable candidates for the high Poisson’s ratio of serpentinized peridotites and pyroxenites. Using new velocity-density relationships for serpentinization at low temperature and high temperature, seismic sections of the oceanic lithosphere from the fast- and slow-spreading ridges are interpreted in terms of composition and serpentinization degree. Ductile shear zones (detachments) tend to develop along serpertinized peridotites and contribute to exhumation of oceanic core complex. Hence seismic profiles of the oceanic lithosphere depend on both composition and serpentinization degree.