Aspects of the kinematic history and mechanisms of superposition of the proterozoic mobile belts of eastern Central Africa (northern Malawi and southern Tanzania)

Abstract

Commonly the Proterozoic tectonic evolution of the Central African basement is subdivided into three major events: (1) the Ubendian (∼2300−1800 Ma), (2) the Irumide (∼1350−950 Ma), and (3) the Pan-African (∼900−450 Ma) orogenic cycles. Relics of the Ubendian event are granite intrusions and, possibly, an amphibolite- to granulite-facies metamorphism. The Ubendian orogeny was followed by deposition of clastic sediments (Muva supergroup). In northern Malawi these sediments and the underlying basement were then thrust to the east-southeast/southeast during the Irumide orogeny. Horizontal shearing along subvertical zones (i.e. the Ubendian belt) was associated with subhorizontal crustal shortening across the Irumide belt. Metamorphism reached the amphibolite facies in the basement and the upper greenschist facies in the Mafingi sediments. Subsequently the metamorphic parageneses were retrograded during ongoing shearing. In Neo-Proterozoic time clastic sediments of the Bukoban, Buanji and Upper Kundelungu groups were deposited. The basement of easternmost Malawi and southern Tanzania was in turn folded and thrust to the northeast onto the Bukoban sediments (early Pan-African orogeny). Deformation occurred under greenschist-facies conditions in northern Malawi and southern Tanzania and reached amphibolite-grade in central Malawi. Eventually, conjugate sets of strike-slip zones cut through the basement. Strike-slip deformation occurred along the retrograde path of metamorphism (late Pan-African orogeny). The structural investigation of the Proterozoic mobile belts shows that these belts represent zones of structural weakness between stable cratons and are very susceptible to various kinds of reactivation. At the subvertical Ubendian belt no structures could be identified which can unequivocally be attributed to the Ubendian orogeny. Major tectonism in the Ubendian belt is expressed by lateral shearing during the later Irumide orogeny. In the course of the Pan-African orogeny and also during Cenozoic rifting the Ubendian belt was again reactivated.