Rifting of the Kalahari Craton through Botswana? New seismic evidence

Abstract

Botswana is a country with relatively low seismic activity that experienced an unexpected Mw 6.5 earthquake on 3 April 2017. Using data from the first countrywide seismic network, we established a Botswana earthquake catalog for the period January 2014 to February 2018. Two areas of elevated seismic activity were detected. The first one is the Okavango Rift Zone in northern Botswana, an area that is known to be active. The other one is associated with the 2017 mainshock and its aftershocks in central Botswana; it follows the Paleoproterozoic suture between the Limpopo Belt and the Kaapvaal Craton. Double-difference relocation of these events revealed a reactivated fault system with a northwesterly strike with the aftershocks occurring at shallower depths than the mainshock at 29 km. The focal mechanisms of the mainshock and selected aftershocks are of normal faulting type with strikes similar to the orientation of the fault system. The unidirectional rupture of the mainshock in the lower crust combined with the upward migration of the aftershocks along the Moiyabana Fault and a thin low velocity anomaly the uppermost mantle are consistent with the events being produced by eclogitization of a dry metastable granulite facies rock by fluid intrusion from the mantle in an extensional stress regime. The Okavango Rift Zone is generally interpreted as the terminus of the southwestern branch of the East African Rift System. We suggest that the recent earthquakes in central Botswana may be considered as the southern continuation of this branch.