- © 2015 Geological Society of South Africa
Malaza et al. (2015) present a rare and important attempt at paleostress analysis of faults in the Karoo Supergroup in the Limpopo Province. Paleostress analyses use field observations of fracture and fault orientations, and inferred fault kinematics, to derive the instantaneous stress field(s) responsible for deformation. Specifically, the method works given that fault slip is a result of shear stress exceeding the shear strength of a fault plane, and the orientation of the fault slip vector equals the orientation of greatest resolved shear stress on the fault plane at the instant of slip (Angelier, 1979). Shear stress is a function of the applied stress tensor and the orientation of the fault plane. Thus, displacement is locally affected by potential reactivation of pre-existing weak faults. If a fault population is studied, however, a regional stress field responsible for the range of observed slip vectors can be derived as long as the faults were active at the same time (Angelier et al., 1982). By ‘at the same time’ we here refer to time in a geological sense, as in the time during which only one stress field was active.
The point we make above is that the stress field at any point in time is a dynamic quantity, and determined by remote forces. The resultant deformation may be affected by inherited weakness zones and structures that are reactivated by the stress field. However, in the brittle crust, frictional sliding on faults leads to stress drops and release of stored elastic energy. As such, stress characteristics are not inherited from past tectonic deformation, although strain characteristics are. Because faults accommodate instantaneous, incremental displacement in response …