Revised Eburnean geodynamic evolution of the ... - Tectonique.net

34 S. Perrouty et al. / Precambrian Research 204– 205 (2012) 12– 39
Fig. 16. Forward-modelled cross-sections across the Ashanti Belt. See Fig. 1 for location of these sections. These models highlight the mineralised Birimian/Tarkwaian
contacts. Sefwi Group metavolcanics were highly deformed during the D1 and contrast with the more open syn-D3 folding in the Tarkwa Basin. Kumasi Group sediments on
the western side of the Ashanti Fault were also deformed by D3. Major gold deposits have been geometrically projected onto their structural position.
anomalies that were not able to be modelled using the previously
described magnetic lithologies. Field observations indicate
the presence of a 20–80 m thick weathered layer at the surface. This
unit corresponds to thick lateritic successions that blanket most
of the study area and contains highly variable magnetic susceptibilities
related to the initial lithology and the degree of alteration
and weathering. This lateritic horizon was modelled as a 50 m
thick homogenous layer. It does not have any influence on the
wavelengths at the scale of our modelling but it does reduce the
amplitude of near-surface features. Given that we see this layer in
the field it would be worse to ignore it altogether, even though we
acknowledge that the thickness variations are unknown. Residual
magnetic anomalies reflect mostly the variations of susceptibility
and thickness within this weathered layer.
Poor gravity coverage over the study area meant these data
were only able to broadly constrain the depth of the relatively
dense Sefwi Group basement. Density values (Table 5)
used during the gravity modelling spanned the range proposed
by Barritt and Kuma (1998) and Hastings (1982), using
3.0 g/cm 3 for gabbros, 2.9 g/cm 3 for basalts and dolerites, 2.8
for Eoeburnean granitoids, 2.7 g/cm 3 for both volcanoclastics,
phyllites and Eburnean granitoids and 2.65 g/cm 3 for the micaschists.
The surficial weathered layer was given a density of
2.6 g/cm 3 .