Paleoproterozoic collision tectonics in West Africa: Insights into the geodynamics of continental growth

Abstract

In order to decipher the collision processes between the Eburnean (Paleoproterozoic) accretionary orogen and its flanking Archean craton, photointerpretation was combined with the extraction of structural data from geological quadrangle maps to draw the structural pattern of the southern West African craton. This pattern reveals multi-scale transpressive anastomosing shear zone networks affecting both the Eburnean orogen and its Archean buttress that absorbed protracted E-W convergence as juvenile crust was still being accreted. Collision involved oblique transcurrent shearing of the craton margin that led to the asymmetrical indentation and escape of the accretionary orogen. As part of the collisional deformation pattern, the craton margin developed a narrow orogenic system recording intense transpressional strain and high pressure-high temperature metamorphism. Lithospheric mantle delamination at the craton margin explains focused heat advection and extreme thermal softening of the crust, enabling steep narrow transpressional collisional orogeny at the rear of accretionary orogens. Delamination also provides a mechanism for the growth of accretionary orogens at the expense of their flanking cratons.