Crustal and Mantle Structure of the West African Craton Beneath SW Niger

Abstract:

The West African Craton (WAC) consists of an Archaean–Proterozoic basement, is covered by sediments ranging in age from Neoproterozoic to present, and is bounded by multiple Pan-African or Hercynian belts. While some parts of the WAC have been studied extensively in recent years, seismic constraints of the central eastern region have hitherto been lacking.

We have analysed recordings from 2005–2013 from the International Monitoring System (IMS) seismic array TORD (Torodi, SW Niger) of the Preparatory Commission of the Comprehensive Test Ban Treaty Organization (CTBTO). Our study provides a good example of how data acquired by the nuclear monitoring community may be used for basic research on the structure and dynamics of the Earth.

Here, we present results of P-to-S receiver function and shear wave splitting analyses that give us insight into crustal, mantle, and mantle transition zone structure. H-Κ-stacking analysis yield bulk crustal properties, with an approx. 40 km thick crust and a low V_p/V_s-ratio of ~1.69. The latter suggests that the Paleoproterozoic crystalline basement is pervasive beneath the array.

At approx. 70 km depth, we detect what we interpret to be a mid-lithospheric discontinuity, which is in good agreement with a recent S receiver function study of the WAC. Both the 410 and 660 km discontinuities are visible in the receiver functions. The mantle transition appears to have a thickness of ~246 km, which is very close to the global average.

Splitting fast orientations show a roughly E–W trend, and delay times are very low (~0.55s ± 0.01s). The results are consistent with recent global azimuthal seismic anisotropy studies. The lack of backazimuthal variation in fast direction indicates a single layer of anisotropy beneath the array. This suggests that the anisotropic fabrics stem from deformation in the Paleoproterozoic craton itself rather than the adjoining Neoproterozoic Trans-Saharan (Pharusian-Dahomeyan) mobile belt, as one would expect the latter to result in stronger anisotropic features.