UNIQUE GEOMORPHIC STRAIN GAUGES TO ASSESS FAULT SEISMOGENIC PROBABILITY IN A REACTIVATED SCR ENVIRONMENT

The late Paleozoic Cape Fold Belt dominates the structural configuration of southern South Africa and formed along the southern margin of Gondwana. Cape Orogeny compression was followed by extension associated with the breakup of Gondwana and the superposition of a Mesozoic extensional system onto the fold belt. The 480 km long Mesozoic fault system includes the major basin-bounding Ceres-Kango-Baviaanskloof-Coega (CKBC) fault system. The river drainages in the region have incised approximately 150 m into the Paleozoic Table Mountain Group quartzite, resulting in the abandonment of high-level pediment surfaces. Geologic mapping reveals the deposits on these remnant surfaces are considered to be part of the Tertiary Grahamstown Formation (Tg) and are composed of fluvial and alluvial fan cobbles and boulders in a sandy matrix that have been cemented with silica and iron (silcrete/ferricrete), making them extremely resistant to erosion. The silcrete and underlying pediment surface at the bedrock alluvium contact are generally considered to be Miocene in age and have exposed surfaces that have been stable for 350 kyr to 3 Myr based on cosmogenic nuclide (26Al/10Be) dating (Bierman et al., 2014), providing unique regional strain gauges for evaluating the presence or absence of fault deformation. Southern South Africa lies within a stable continental region (SCR); however, evidence of up to three Pleistocene to Holocene normal faulting events along a 100 km reactivated portion of the Kango fault has been documented, including 10-33 m of normal dip slip of the pediment surfaces (Hanson et al., 2014). To determine the seismogenic probability of major neighboring faults in the CKBC fault system, geologic mapping along pediment surfaces that overlie the Baviaanskloof, Western Coega, and Kouga faults was undertaken. Detailed mapping of the bedrock fault locations and the silcrete unconformity, and the silcrete surface itself reveals a demonstrable lack of surface rupture along these faults, thus confirming the lack of Quaternary reactivation along most of the CKBC fault system. The mapping studies provided data used to evaluate the seismogenic probability of these faults for seismic hazard analyses and they provide insights into the spatial and temporal reactivation of faults within SCR.