A palaeoseismic trench investigation of early Holocene neotectonic faulting along the Kango Fault, southern Cape Fold Belt, South Africa – Part I: stratigraphic and structural features

Abstract

Who would have thought that the southern part of ‘stable, old South Africa’ was in fact ‘not-so-stable’, and that there has been spectacular tectonic activity in the Cape Fold Belt as recent as the early Holocene, about 10,327 ± 755 years ago? Large-scale surface rupture, with at least 2 m of vertical geomorphic offset, has been recorded along the Kango Fault over a lateral distance of at least 84 km, from near the town of De Rust, east of Oudtshoorn, towards the western end of the Baviaanskloof in the Eastern Cape. Although the Kango Fault scarp was reported in the mid-1970’s and late-1980’s, detailed palaeoseismic investigation was not undertaken until recently, when the chief energy supplier for South Africa, Eskom, rekindled investigations to establish a number of nuclear power plants along the South African coastline. This paper reports the results of the first palaeoseismic trench excavated across the Kango Fault during the initial phase of the nations’ current New Build programme. The location and date of the most recent event (MRE) is reported, as well as the style of faulting and local stress direction. These palaeoseismic data are derived from carefully-mapped logs of an 82 m long, 5 m deep trench, supported by 12 optically-stimulated luminescence (OSL) dates of stratigraphic horizons exposed in the trench. Various features of the stratigraphy, pedology and structure in the trench sidewall are described in detail, particularly those leading to the interpretation of the MRE. Development of a shear fabric in the cobble- to small boulder-sized alluvium at the main fault zone indicates that interpretation of local neotectonic fault style and stress direction should be made at least 4 to 5 m below the free face of the fault scarp. Retro-deformation of the trench log clearly defines the MRE and distinguishes apparent vertical displacement from the actual displacement of 2.0 ± 0.06 m. It also reveals that 1.9 m of surface extension occurred during the rupture, as well as the depositional processes that subsequently buried the 32 m wide graben formed at the site.