MAGMATIC DIKING AND UNDERPLATING BENEATH THE HOGGAR SWELL, ALGERIA REVEALED BY P-TO-S CONVERSIONS

One of the tectonically most intriguing features in northern Africa is the widespread distribution of Cenozoic volcanic centers or massifs, such as the Hoggar swell in Algeria. A general lack of high-resolution geophysical studies of the crust and mantle beneath the massifs is mostly responsible for the heated debates about the depth of the source region of the Cenozoic volcanism and the closely related uncertainty about the mechanism that formed and maintains the high elevation of the swells. In this study, we report results from a systematic study of 1386 high-quality receiver functions recorded by station TAM, the only permanent broadband seismic station on the Hoggar swell. The resulting crustal thickness is about 34 km and the Vp/Vs is 1.77 when all the receiver functions from the station are stacked. Our study reveals a sharp contrast in the amplitude of the P-to-S converted phases between the volcanic, highly-fractured Tefedest terrane and the non-volcanic, less fractured Laouni terrane. The former has a stacking amplitude that is comparable to typical cratonic areas, and the latter has an amplitude that is only about 25% as large. Spatially consistent crustal thickness and an intermediate-mafic crust are inferred on the Tefedest terrane, while spatially variable crustal thickness and a felsic crust are inferred beneath the Laouni terrane. The observations can be best explained by a mantle-derived underplated magmatic layer beneath the mechanically-stronger Laouni terrane, and magmatic diking and resultant volcanism associated with the mechanically weaker Tefedest crust.