Transpressional flow and escape of orogens: The Paleoproterozoic Eburnean accretionary orogen of West Africa

The kinematic interpretation of natural transpressional deformation remains limited by strain partitioning in three dimensions. This prompts multiscale data integration to extract tectonically meaningful information on the role of strike-slip fault systems in orogen building processes. Here, we investigate the kinematics of mid-crustal deformation in an archetypal wide transpressional orogen from the mesoscale (kilometers) to the regional scale (∼1000 km), based on an exhaustive compilation of structural data, the construction of an integrative structural map, and the computation of the orogen's strain field. We document a pervasive crustal flow mode, termed bulk inhomogeneous transpressional flow, which accommodates convergence through a self-similar anastomozed fabric–shear zone network that converts crustal shortening into lateral and vertical escape. This flow pattern may exhibit local complexity due to partial annealing or recovery of shear zones and fabrics during a prolonged (tens of Myr), cumulative deformation history. As a result, local kinematic observations or relative deformation chronologies within the structural network may not capture bulk flow and thus have limited tectonic significance. Transpressional flow is pure-shear dominated and produces distributed crustal thickening combined with orogen-parallel escape, the latter involving a component of extension. Regional flow patterns follow the convex shape of the boundaries of stiff crustal domains and oroclines. Distributed flow implies crust–mantle coupling facilitated by lithospheric mantle attenuation beneath the orogen. Orogen-scale transcurrent shear zones primarily act as transfer structures accommodating flow heterogeneities rather than plate-scale displacements. Crustal flow therefore leads the kinematics of wide transpressional orogens and must operate in the hot portions of most orogens. These results challenge fault-driven escape tectonics models and support vertically coupled, distributed deformation across lithospheric layers.