Ductile to Brittle Progression During Exhumation Controlled the Architecture of the Naukluft Nappe Complex, Namibia

Abstract

Mountain belt architecture and evolution can be represented by frictional Coulomb wedges, a simplification that works well in the shallow deformation conditions of many foreland fold‐thrust belts. However, thin‐skinned fold‐thrust belts often occur in rocks that can deform ductily at low temperature. We demonstrate the importance of early ductile deformation in establishing the regional structure of a foreland thrust belt. In the Naukluft Nappe Complex (NNC) in central Namibia, the earliest major shear structures are 10 s meters‐thick mylonite zones in greenschist‐facies carbonate rocks. These evolved to brittle faults as in‐sequence thrusting and imbrication resulted in exhumation of the older structures. Embrittlement caused imbrication resulting in more classical thin‐skinned architecture toward the foreland. Cooling stopped further carbonate mylonitization and shifted the deformation into shales and along contacts. Our 40 km2 map shows sequential evolution of each nappe‐bounding structure from ductile to frictional, superimposed on the exhumation of the belt. The final architecture is representative of this evolution, with low angle, long wavelength structures toward the hinterland and steeper imbricate sets of faults and folds toward the foreland. Mylonite zones and other internal deformation were not accounted for in previous shortening estimates. We negate some of the previously proposed evidence for shortening, and estimate ≥150 km of translation and internal shortening is recorded in the NNC. We demonstrate that even “thin‐skinned” orogenic belts may be strongly influenced in their architecture by ductile deformation, particularly in the early stages of development, and that this deformation must be accounted for in tectonic reconstructions.