Lithospheric anisotropy on the Kerguelen hotspot track inferred from Rayleigh wave polarisation anomalies

Abstract

Rayleigh waves recorded at the Geoscope station PAF on the Kerguelen Isles in the Indian Ocean, show strong polarisation anomalics in the period range 20-50 s, as demonstrated by dispersion analysis of 3-component recordings. The largest and most consistent anomalies are observed for events located in the southern part of the Java Trench. At 25 s the Rayleigh waves present transverse components with an amplitude of up to 55 per cent of the amplitude of the longitudinal components. The particle motion in the horizontal plane is largely elliptical. By comparison, very few and mostly small polarisation anomalies are detected at the nearby Geoscope stations AIS and CRZF on the Amsterdam and Crozet Isles, respectively. Wave path deviations from the epicentre-receiver great circle, as calculated in tomographic models of the Indian Ocean, cannot explain the polarisation anomalies. Using a multiple-scattering scheme for modelling surface waves in 3-D heterogeneous and anisotropic structures, we show that wavefield distortion due to the geometrical structure of the Kerguelen Plateau in the vicinity of the station cannot explain the anomalies either, but that anisotropy can. We infer the presence of an anisotropic structure in the lithosphere to the north of the Kerguelen Isles, containing 40 per cent oriented pyrolite, with fast axis tilting downwards in a north-north-east direction. The anisotropy may be caused by deformation of the lithosphere related to the Kerguelen hotspot.