Lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton: interplay of coeval contractional deformation and extensional gravitational collapse

We present new litho-structural and metamorphic maps of the Paleoproterozoic (2.25-2.00 Ga) West African Craton in northern Ghana, based on the interpretation of field observations and airborne geophysical datasets. The study reveals contrasting metamorphic domains consisting of high-grade gneisses and low-grade volcano-sedimentary belts, separated by shear zones and assembled during the Paleoproterozoic Eburnean orogeny (2.15-2.07 Ga). Supracrustal rocks were buried and metamorphosed at conditions corresponding to the amphibolite-granulite facies transition, during a (D1) deformation event, consistent with N-S horizontal shortening, which generated reverse shear zones. High and low metamorphic grade rocks were brought in contact along extensional shear zones formed during N-S extension (D2). These structures were overprinted by contractional deformation associated with E-W shortening and N to NNE stretching (D3), coeval with amphibolite-facies metamorphism. The late-stage tectonic evolution (D4-D7) is characterised by strain localisation in successive generations of narrow shear zones, and the re-activation of inherited structures in a dominantly transcurrent regime. U-Pb dating of zircon and monazite from magmatic and metamorphic rocks reveals that D1-D3 deformations form a continuous and overlapping time sequence between ca. 2140 and 2110 Ma. The changes in deformation style from D1 to D3 are interpreted to reflect a shift from dominant horizontal tectonic forces to an interplay between tectonic and gravitational forces, which allowed for the exhumation of the lower crust in an anatectic migmatite dome. We suggest that doming is accommodated by lateral extensional sliding of the upper crust and amplified by coeval orthogonal shortening. The abrupt rotation of shortening directions points to a change in boundary conditions applied to the orogen. We hypothesise that it is due to the collision of northern Ghana with the Paleoproterozoic province in modern-day south-western Burkina Faso, which shows a contrasting geological history. The results reveal that the Eburnean orogeny in NW Ghana shared some thermo-mechanical similarities with modern orogenic belts. The findings bring new insight in Paleoprotezozoic plate tectonics, at the transition between archaic and modern geodynamics.