ALBERT

Description: It is well known that the Seychelles microcontinent resulted from episodic rifting between Africa and India. However, the mechanics of such rifting are poorly understood. The aim of the SEISM project (Seismic Experiment to Investigate the Seychelles Microcontinent) is a better understanding of microcontinent formation through a seismic study of the upper-mantle and crustal structure beneath the Seychelles. From Feb. 2003 and Jan. 2004, 8 broadband and 18 intermediate-period three-component seismometers were deployed on 18 islands in the Seychelles. During this time period 240 events of Mb $〉$ 5.8 were recorded. Microseismic noise was a problem on most islands and resulted in low signal to noise ratios in seismic recordings. A polarisation filter developed by Du et al. (GJI, 2000) has been used to enhance teleseismic signals. The first stage of data analysis has been a study of upper-mantle anisotropy using shear-wave splitting in core phases such as SKS. In general the degree of splitting is similar to global averages (i.e., $sim$1 second). Variability in the orientation of the anisotropy suggests a mechanism related to deformation associated with microcontinent formation during the breakup of the Madagascar/Seychelles/India land mass, rather than a mechanism associated with current plate motions. The granitic inner-islands of the Seychelles plateau show a coherent NNE-SSW trend in anisotropy, whilst stations near the Mascarene plateau show more E-W trends. Such trends may support ideas of an anticlockwise rotation of the Seychelles during rifting events, or influence from local plume upwelling. The boundaries between oceanic and continental parts of the plateau are poorly know. Crustal thicknesses can be used to determine oceanic and contenental parts. Receiver functions will be used to estimate crustal depths and deeper mantle discontinuities.