POLYPHASE INVERSION OF AN INTRAPLATE RIFT-BOUNDING FAULT—THE WATERBERG THRUST IN NAMIBIA

The Waterberg thrust is an enigmatic thrust fault in northern Namibia that brings late Precambrian Damara (Panafrican) basement rocks up against Triassic and lower Jurassic Karoo clastic sediments of the Waterberg Basin. It trends NNW between two of the major regional lineaments that cut across northern Namibia, which run subparallel to, and are rooted in the grain of the basement Damara orogenic belt. Mapping by a team of geologists from Universität Würzburg has been used to construct and restore several sections across the thrust. Those sections and on-the-ground studies indicate that motion was initiated contemporaneously with Karoo deposition, forming unconformities within the Omingonde (Triassic) sediments and the Etjo (Jurassic) sandstones. The unconformities were subsequently tilted and folded during continued motion on the fault as well as involved with local coarse sediments derived from the basement in the hanging wall. The progressive contractional deformation initiated in late Karoo time with episodic progression during the development of the basin, and continued subsequently during Cretaceous uplift of the area. Insofar as this structure occurs along a trend of Karoo-aged extensional basins, the thrust is inferred to be an inversion feature even though hanging-wall thickening of a syn-rift section cannot be proven. A similar structure occurs to the west in the Otongundu Basin where it is much more deeply eroded and only remnants of the footwall Karoo remain. These extensional basins contain an array of basement blocks, all entrained between the reactivated lineaments functioning as inter-basinal transfer faults. This situation suggests that in this case the inversion is generated by changes in the rate of relative block motion, rather than any regional change in 'stress directions.' Instead of invoking a vaguely separate tectonic event, this instance of inversion can be easily explained as a natural development in the kinematic history, which we call syn-kinematic inversion.