Assessing Recency of Faulting in the Stable Continental Region of Western Cape, South Africa

ABSTRACT Field and desktop mapping studies were conducted for the stable continental region in the Western Cape Province of South Africa to characterize fault activity of four fault systems, including the Worcester, Groenhof, Piketberg-Wellington, and Colenso faults. The geologic studies presented here were in support of a Probabilistic Seismic Hazard Analysis (PSHA) for a nearby nuclear power facility site. Previous studies performed by the South African Council for Geoscience in the region suggested evidence of near-surface co-seismic deformation (De Beer, 2004; De Beer et al., 2008). The goal of this study is to re-assess the prior interpretations of these four faults and gather the required data for including them in a seismic source model for use in a PSHA. The primary aspects to include in the characterization are the recency of movement, slip rate, kinematics, and geometry. To improve the interpretation and target sites, the study used a satellite-derived digital elevation model and aerial imagery for six areas, totaling over 900 km2 of data. Limited Quaternary cover, or other late Cenozoic deposits that overlie the Precambrian and Paleozoic bedrock structures, resulted in difficulty constraining the recency of faulting. The new observations presented in this study suggest that reactivation and surface rupture along pre-Cenozoic faults of the four fault systems have not occurred in at least the last 10 ka. Further, the lack of youthful tectonic geomorphology and deformation of Quaternary stratigraphy indicate that surface faulting has not occurred in the late to middle Quaternary along any of these four structures.