TELESEISMIC INVESTIGATIONS OF THE HOGGAR SWELL, ALGERIA

The Hoggar Swell in Algeria is one of the significant massifs of central Africa. Those massifs have similar elevation and dimensions and are characterized by volcanism of Eocene to Quaternary in age. The paucity of high-resolution geophysical studies of the crust and mantle beneath the massifs is largely responsible for the heated debated about the plume versus non-plume origin of the domal structures. Here we present results of crustal thickness, crustal Poisson’s ratio and sharpness, shear-wave splitting, and mantle transition zone thickness measurements using 20 years of broadband seismic data recorded by station TAM located on the Hoggar Swell to constrain the models for the formation of the Swell. Stacking of about 1300 P-to-S receiver functions revealed a mean crustal thickness of 32 km, which is similar to the thickness of a ‘normal’ cratonic crust, and a Vp/Vs of 1.79, which corresponds to a Poisson’s ratio of 0.27 and represents a slightly more mafic crust than a typical cratonic crust. The Moho is significantly blurred, probably due to a mantle-derived layer beneath the Moho. About 120 SKS, SKKS, and PKS splitting measurements were obtained at the station. Those measurements show insignificant azimuthal variations, suggesting a single layer of anisotropy. The fast direction is dominantly N-S which is consistent with the strike of the megafaults in the area, and is inconsistent with the predicted radial pattern associated with a mantle plume. The splitting time is 1.0 +- 0.3 s which is similar to the stable Kaapvaal craton and represents an approximately 100 km thick layer of anisotropy. Stacking of about 1100 moveout corrected receiver functions suggests that the depth of the ‘670’ km discontinuity beneath the station is 675 km and is well defined, suggesting the absence of hot material at the bottom of the mantle transition zone (MTZ). On the contrary, that of the ‘410’ km discontinuity has two peaks located at 432 and 452 km, respectively, resulting in a MTZ thickness of 243 and 223 km, respectively. The latter thickness suggests a positive MTZ temperature anomaly of about 140C. Our results are more consistent with the intraplate reactivation model than the plume model for the origin of the Hoggar Swell.