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- Title
Rifting of the Kalahari Craton Through Botswana? New Seismic Evidence.
- Authors
Paulssen, H.; Micallef, T.; Bouwman, D. R.; Ruigrok, E.; Herman, M. W.; Fadel, I.; van der Meijde, Mark; Kwadiba, M.; Maritinkole, J.; Ntibinyane, O.
- 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. Plain Language Summary: Earthquakes do not happen very often in Botswana, but an unexpected magnitude 6.5 earthquake occurred on 3 April 2017. We investigated earthquake occurrence in Botswana for the period January 2014 to February 2018 using data from the first countrywide seismic network. We found that the magnitude 6.5 earthquake in central Botswana was located deep in the crust and that the aftershocks were all at shallower depths. The main‐ and aftershocks were located along the boundary between two cratonic blocks that were welded together 2.6 Gyr ago. The earthquakes show that this fault zone is now reactivated in a stress regime that leads to rifting. It is likely that the earthquakes are caused by fluid infiltration from the mantle into the crust, altering the rock of the ancient fault zone. It is generally thought that the seismically active Okavango Rift Zone in northern Botswana is the (southern) end of the southwestern branch of the (continental scale) East African Rift Zone. We suggest that this branch has progressed southwards through central Botswana. Key Points: New Botswana earthquake catalog indicates rifting along Okavango Rift Zone and Limpopo BeltLower crustal earthquakes triggered by mantle fluids and eclogitization of dry metastable granulite rockSouthwestern branch of East African Rift System may continue through central Botswana
- Subjects
BOTSWANA; EARTHQUAKE aftershocks; EARTHQUAKE magnitude; SEISMIC networks; EARTHQUAKE zones; RIFTS (Geology); FAULT zones; EARTHQUAKES; FACIES
- Publication
Journal of Geophysical Research. Solid Earth, 2022, Vol 127, Issue 4, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2021JB023524