Cratons being the old and stabilised portion of continental lithosphere that had strongly defied and survived the regional deformation for a very long period (Jordan, 1978). These regions, primarily of Precambrian age (Goodwin, 1996), existing in almost all the continents, with the Archean nucleus as stable blocks in the center, while the margins are deforming (Bleeker, 2002). These tectonically inert/stable blocks, free of any recent activity, thus preserve old crustal material within. Investigating and understanding the crustal architecture of these stable blocks can thread our knowledge of the old lithospheres’ evolution through formation, deformation, and destruction. Growing research focused on the Precambrian structure in the most recent years has diverged from the long-held concept of crustal thickness directly proportional to age (Meissner, 1986; Smithson et al., 1981). These studies estimating depth-velocity structure have compiled a thin Archean crust in contrast to the thick Proterozoic crust (Durrheim and Mooney, 1991; 1994; Thompson et al., 2010). The proposed cause of thicker crust is linked to a thick high Vs (~ 4.0-4.3 km/s, as in Kachingwe et al., 2015), mostly associated with mafic lithologies forming the lowermost crust in the Proterozoic structures. However, in older Archean crust the lower crust is absent or depleted (Abbott, 1991; Rudnick, 1995).
Crust is argued to have formed either through horizontal/vertical volcanic arc accretion (Kusky and Polat 1999; Lowe, 1994) or through frequent interaction and placement of magmas from mantle plume (Choukroune et al., 1997) forming distinctive layers in the continents. This continental crust has mafic lower crustal composition, a geochemical composition that of arc magma above subduction zones and thus argued to have been generated from relamination process (Kelemen and Behn, 2016; Rudnick, 1995). This most discussed mafic lithology is argued as result of episodic depositions of basaltic magmas (Nelson, 1991) causing thickening of the crust and that posed to reworking as magmatic differentiation during stabilization, under high heat-flow. This highly mafic composition settled at the crust base as the lowermost crust, got depleted by lateral delamination in the Archean age put under further reworking. However, various other models diverge from this concept. Few studies found no significant differences in the Precambrian age structure (Rudnick and Fountain, 1995) and suggest no direct correlation of structure with the basement age (Kennett et al., 2011). While few other studies found no alteration of Precambrian structure, since their stabilization in the Archean age (Tugume et al., 2013; Kachingwe et al., 2015), and a few studies report reworking of continental crust during Proterozoic time (Drury et al., 1984; Julià et al., 2009). Recent studies of xenoliths in the Archean terranes compiled by Rudnick and Gao (2003) insist on the underplating of the Archean crust of continents in the post-Archean times. These differing models proposed explain the need to further study the continents. All these validations come from direct and primary evidences from the lithospheric velocity-depth structure. Thus, mapping Vs and nature of associated composition at depth further provides high importance in refining our understanding of the crust of the oldest parts of continents.
The crustal structure secular variation in the Precambrian domains is often determined by visible thickness and compositional differences in the crust, which is linked to their period of formation and stabilization. While the shallow (~ 32–39 km) and sharp Moho interface defines the interiors of old Archean cratonic domains, a comparatively deeper and diffusive/gradational Moho interface define the young Archean or Proterozoic domains (Yuan, 2015) which form the boundaries of these old domains. The velocity layers at depth are often related to bulk composition describing the nature of sub-surface structure interpreted as felsic/intermediate/mafic. Like the crustal thickening is often added to existence of high velocity composition forming the lowermost crust. Addition of a thick layer of high velocity at lower crust, thickens younger Precambrian crust, and modifies the bulk composition to intermediate-to-mafic, while the removal of the layer from the lower crust of older Acrhean crust, thinning the crust and averages bulk composition change to felsic.
Various studies have brought a global correlation of continental crustal structure, and in its tectonic, geodynamic settings, which has put forward few models discussing their evolution based on their structure, their physical, geo- and thermo-chemical properties (Artemieva and Shulgin, 2019). However, this variation is not distinctive and clear, but an important factor in understanding the continental crust, and to link to its evolution.
The age of West Africa Craton (WAC) makes it one of the oldest cratons of Africa, which remained stable since 2.2-2.0 Ga. Very limited seismological studies are available on crustal structure in the south of the WAC. Previous studies with receiver function (RF) (Akpan et al., 2016; Ammon et al., 2004; Sandvol et al., 1998) either used limited number of seismic data or averaged all the data to provide average crustal structure beneath a few stations. To improve the understanding of lithospheric structure in the WAC, a significant quantity of RFs with high signal-to-noise (S/N) have been used. RFs in all the possible directions are then joint-inverted with Rayleigh wave group velocity dispersion data. The main focus of the current work is to estimate the depth and nature of Moho (sharp or diffuse), and the bulk composition of crust, especially the lower crustal structure (thickness and Vs) in the study region.