Deformation in carbonates and its contribution to fold & thrust belt architecture

Fold and thrust belts are frequent structures that form under a compressive regime. They record a variety of deformation that guides earthquake ruptures and controls seismic hazard. The Naukluft mountains of Namibia with their extraordinary exposure, present a unique opportunity to study carbonate fault rocks. Through linked field investigations and microstructural analysis, we aim to map how changes in fault strength contributed to larger scale architectural evolution of a fold and thrust belt. Mineralogical assemblage of the stacked nappes show units of carbonate and shale in the lower to upper greenschist facies, correlating to the seismological depth for carbonate rocks. Cross cutting relations prove that the Naukluft Nappe Complex (NNC), previously thought to form through gravity sliding, is a foreword-propagating fold and thrust belt, stacking Neo Proterozoic sedimentary nappes on top of each other, and on top of the cambrian Nama fore-rift group. Most thrusts separating the stacked nappes are made of 20-150 meters of calc mylonites, except for the youngest two brittle thrust faults, including the basal thrust. Brittle deformation is recorded by localized brecciation and the development of discrete faults. The brittle faults follow the same orientation as the overall transport direction of the nappe complex. Ductile shearing is facilitated by crystal plastic deformation, leading to grain size reduction, and grain boundary sliding. Both ductile mylonite units and the younger brittle faults are also overprinted by later brittle faults, and between the ductile and brittle behavior, we map a unit of a block-in-matrix. We found no clear correlation between the temporal evolution of the nappe complex and the thickness of ductile shear zones. We show evidence for embrittlement through exhumation of a carbonate terrane, and suggest that it may be important to consider the effects of deeper ductile behavior in “Coulomb wedge” descriptions of thin-skinned thrust belts.