Multiple planation surfaces in basement regions: Implications for the reconstruction of periods of denudation and uplift in southern Zimbabwe

This study attempts to examine the competing hypotheses, whether planation surfaces separated by scarp zones arise from differences in lithology and structure or are directly linked with episodic uplift and long periods of tectonic quiescence, as is supposed in conventional geomorphic concepts. In these concepts, variations in lithology and structure and climatic changes do not divert the general course of landscape development.Multivariate statistical analysis of morphometric parameters indicates a strong association between lithology and scarp-zone terrain types. The variability within petrographically homogeneous areas is determined by structures such as fractures that are exploited more easily by streams, whilst on granitoid rocks in particular, differences in terrain types appear to result from a combination of differences in fracture density and the differing susceptibility of the rock to chemical weathering in moist and dry environments. These factors promoted the development of scarps and knickpoints that became structurally/lithologically controlled base levels and act as barriers for the continuous transmission of base-level changes caused by tectonic uplift or by sea-level lowering. Accordingly, the surface sequence of the study area is likely to be entirely of denudational origin and may be virtually unrelated to episodic changes in base level.Numerical process–response modelling indicates that in lithologically/structurally variable terrains, an initial perturbation may induce diverging landscape-developmental trends. Local adjustments of denudation and river downcutting rates on structurally/lithologically controlled base levels and the differing coupling strength of landscape elements to base-level changes may obscure both the direct links between the number and magnitude of perturbations and the distinction between continuous and intermittent vertical displacements. This implies that the vertical separation between planation surfaces does not necessarily indicate episodic vertical tectonic displacement or sea-level change. According to the complex array of factors influencing the development of planation surface sequences, evolutionary models relying on a synchronous continent-wide formation of planation surfaces appear to be untenable in lithologically and structurally variable basement regions.