An olivine fabric database: an overview of upper mantle fabrics and seismic anisotropy

Abstract

We present a unique database of 110 olivine petrofabrics and their calculated seismic properties. The samples come from a variety of the upper mantle geodynamic environments (ophiolites, subduction zones, and kimberlites) with a wide range of micro-structures. A phenomenological relationship is established between P- and S-wave seismic anisotropy and the degree of crystal alignment (fabric strength). Seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20% for P-waves and 10 to 15% for S-waves. Despite a large variation in the symmetry of fabric patterns, the average seismic properties of the different fabric, micro-structural and geodynamic settings have similar anisotropies in both magnitude and symmetry. Hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known, but not geodynamic environment or details of the petrofabric pattern. A simple pattern of seismic anisotropy characterises the average sample of the database, which has the following features: the polarisation plane of the fastest S-wave is parallel or sub-parallel to the foliation plane; the maximum shear wave splitting is parallel to the Y structural direction (in foliation plane and normal to the lineation); the maximum of the P-wave velocity is parallel to the high concentration of [100] axes, which is sub-parallel to the lineation. The [100] orientation distribution has the greatest influence on the P-wave seismic anisotropy. The [100] and [001] orientation distributions have the greatest influence on the symmetry of S-wave anisotropy, although the magnitude of anisotropy is influenced by the distribution of all three principal axes. Although the database only contains olivine petrofabrics, this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle.