Applicability of thin or thick skinned structural models in a region of multiple inversion episodes; southern South Africa

Abstract

The deformation of the Cape Fold Belt has been attributed to repeated structural reactivation of a mega-detachment from the late Proterozoic to the Mesozoic (650–65 Ma). Through the integration of onshore cross-sections with observations from the offshore Mesozoic extensional system this study evaluates the applicability of the mega-detachment model. Regional scale cross-sections through the Permian-Triassic Cape Fold Belt reveal that it comprises two main structural domains: a northern domain dominated by northward verging and asymmetric folds; and a southern domain comprising a series of approximately 8 km wavelength box folds. The genesis of these box folds is attributed to motion on underlying high angle (>45°) reverse faults. This variation between north and south in the fold belt is reflected by a similar variation in extensional geometry of the Mesozoic normal faults, as revealed by subsurface data. The normal faults demonstrate a progressive increase in dip from 24° in the north to 60° in the south. Features commonly attributed to thin- and thick- skinned tectonic models are observed in both domains, therefore it is not appropriate to describe the observed deformation as one of the two end members. In addition, the structures are inferred to have undergone at least two stages of inversion, irrespective of dip. This is not predicted by either end-member model.