A laser-probe 40Ar/39Ar study of pseudotachylite from the Tambach Fault Zone, Kenya: direct isotopic dating of brittle faults

Understanding the tectonic evolution of orogenic belts and intracratonic areas depends on our ability to determine the age of tectonic features on a variety of scales. This study demonstrates the value of the laser-probe 40Ar/39Ar dating technique, which, if applied to fault-derived pseudotachylites, may be used to directly determine the age of brittle faults. The laser-probe technique affords high spatial resolution, enabling a greater opportunity for discriminating between pseudotachylite matrix, host-rock clasts and alteration products that are often present in varying proportions within pseudotachylites. The laser-probe 40Ar/39Ar technique has been applied to pseudotachylite samples from the Tambach Fault Zone (TFZ), a major NW–SE trending strike-slip fault within the Kenyan part of the Late Proterozoic/Early Palaeozoic Mozambique Belt. The pseudotachylites of the TFZ were previously thought to have formed either (i) at about 530–430 Ma, or (ii) during the Cenozoic evolution of the Kenya Rift. In the latter case, seismic slip on the rift-bounding normal fault would have generated the pseudotachylites, due to the reactivation of old NW–SE trending structures in the basement. Based on our new data, we interpret the pseudotachylite formation age to be 400 Ma. This rules out the possibility that the pseudotachylites are related to the formation of the Kenya Rift. Although the inherited basement faults may have been locally reactivated as transfer faults, reactivation of these structures during rifting did not occur beyond the margins of the Kenya Rift.