STRATIGRAPHIC, STRUCTURAL AND TECTONIC ENIGMAS ASSOCIATED WITH THE CAPE FOLD BELT: CHALLENGES FOR FUTURE RESEARCH

Abstract

The Cape Fold Belt has been a topic of investigation for over 100 years, and with each renewed surge of interest, new ideas on the origin of the fold belt as a whole have come to the fore. During the last two decades researchers who have focused on the structural geology of the Cape Supergroup, in particular, have provided greater insight into local characteristics of the fold belt, but a broader understanding of the tectonic development of the fold belt is not without controversy. Research focus has been on documentation and interpretation of mainly thin skinned structural characteristics in the central and eastern part of the main “arm” of the fold belt. Identification of thrust sheets, particularly in lower units of the Cape Supergroup coincident with the thickest accumulations of sediment, led researchers to propose a thrust stacking mechanism to account for the abnormal thickness of quartzitic units, rather than previously accepted models. The significance of thrust stacking and the disruptive effect it has had on sediments of the entire Cape Supergroup is emphasized. The stratigraphic order of Cape Supergroup rocks, as currently accepted, is therefore called into question, especially in areas of pervasive thrusting, and needs revision. Some tectonic models proposed to date account only for local characteristics of the fold belt, whereas other models do take into account the entire fold belt. Examples of the latter include the Andean type model which adequately explains the general northward vergence of structures, but thickness estimates of stratigraphic packages of Cape and Karoo Supergroup cover rocks revealed through recent seismic surveys across the central part of the fold belt do not corroborate this model. Proponents of the Transpression model, on the other hand, advocate that right-lateral strike-slip motion produced en-echelon folds and flower structures in some parts of the fold belt. The latter model lacks refinement in that it does not explain the presence of structures interpreted to have formed through left-lateral movement, especially in the western part of the fold belt. This paper outlines unanswered questions relating to the stratigraphy, structural geology and proposed tectonic models that remain enigmas, and these need resolving before the tectono-thermal history of the fold belt can be fully understood.