DUCTILE TO BRITTLE DEFORMATION LOCALIZATION, EMBRITTLEMENT AND FLUID-ROCK INTERACTION DURING THRUSTING AND EXHUMATION OF A THIN-SKINNED NAPPE COMPLEX: NAUKLUFT, NAMIBIA

Continental thrust systems are responsible for mountain building and associated earthquake hazard. Over geologic time, the balance between ductile deformation at depth and brittle deformation near the surface, and the links between brittle and ductile structures, contribute to the fault/fold architecture that guide earthquake ruptures and control seismic hazard. To understand the interplay between brittle and ductile deformation, we investigate the architecture and deformation history of an extremely well-exposed klippe. The Naukluft Nappe Complex (NNC) is of a Cambrian age. It is exposed in the Naukluft mountains of Namibia and is composed of stacked Neoproterozoic sedimentary and metasedimentary units. Studies of the NNC have focused either on identifying the ‘root zone’ of the nappes, or on the specific deformation mechanisms of the basal thrust, which was originally interpreted as a gravity slide in the mid-20^th century. In this study, we use detailed field mapping, fault rock studies and kinematic models to examine the emplacement of this klippe. Over an area of 4 by 4 kilometers, five faults and shear zones have been mapped. Two dolo-mylonites occur along the edges of thick, apparently rigid dolostone units which show internal fault imbrication. These mylonites are cut and brecciated by co-planar faults, indicating progressive localisation and embrittlement during thrusting. Shale units were thickened by pervasive internal imbrication and folding revealed by mapping of internal carbonate marker beds. Solution-precipitation played a major role in changing rock composition during deformation, introducing quartz into carbonate units, and exchanging dolomite for calcite, which affected rheology as thrusting exhumed the units. We present evidence for in-sequence thrusting of this nappe complex, in a process combining thrusting and exhumation, resulting in gradual embrittlement of the major structures. Both strong and weak rocks participated in deformation, suggesting that movement on faults was not only controlled by availability of weak lithologies. A kinematic restoration model will be built based on the structure geometries to access the structure chronology, providing a thorough insight for fault exhumation, and thrust evolution in compressional orogens.