Tectonic History of the Kaapvaal Craton: Constraints From Rheological Modeling

Abstract

In this study, the lithospheric strength of the Kaapvaal Craton is determined using deformation laws describing the failure of rocks at lithospheric depths. Shear wave velocity results from a previous study were used to constrain the rheological profiles associated with each of the 64 broadband seismic stations deployed across the craton. The other constraint, are the geotherms estimated assuming steady state conditions. In order to address the random and complex nature of uncertainties inherent in geotherms at lithospheric depths, a Monte Carlo simulation approach is used to find well‐constrained geotherms for each of the terrains within the craton. The optimized geotherms were subsequently used to determine the rheological profiles associated with each of the 64 localities. The resultant rheological profiles show seismogenic depths of 5–16 km for the craton and further show that the craton can basically be delineated into three strength domains. Most parts of the Kimberley terrain and minor parts elsewhere in the craton show a mechanically weak crust with a strength ratio less than 0.5, a substantial portion of the Witwatersrand terrain shows moderate strength ratios between 0.5 and 1.0, and the rest of the craton shows a mechanically strong crust corresponding to strength ratios greater than 1.0. Strength ratio is an indication of the extent of lower crustal to mantle strength. Low strength ratio regions correlate with parts of the craton that had a significant deformation history while high strength ratio regions correlate with a stable lithosphere that had minor or no deformation history.