Precambrian collision tectonics in Africa

Abstract

Summary: Stages of the collisional process are separated. Criteria are proposed by which A- and B-subduction may be distinguished. Late Precambrian structures in NE Africa are attributed to successive gentle collisions of island arcs; the sutures dip E or SE and relative plate motion was NW-SE. In the polycyclic Proterozoic Mozambique Belt of E Africa there are probably three sutures (Sekerr, Baragoi, Moyale-Shakiso) with ophiolites; they appear to dip E. It is argued that despite the lack of geochronological evidence, Archaean basement probably underlies the metasedimentary sequences, right across the Mozambique Belt of E Africa, from the Archaean Tanzania Craton on the W to the Archaean Craton, represented by Madagascar and S India, on the E. The belt is therefore interpreted as having been built by collision of continental plates. Widespread recumbent structures, complex and intense deformation and high-grade metamorphism imply crustal thickening and a multistage collisional history. Relative NW-SE motion was probably followed by N-S post-collisional ductile shear. The Archaean Limpopo Belt in southern Africa shows evidence of drastic crustal thickening and intense deformation in contrast to the much weaker deformation and little crustal thickening in the Kaapvaal and Rhodesian Cratons to the S and N. Collision is implied but in the absence of convincing ophiolites, subduction-type calc-alkaline magmatism and abyssal sediments, B-subduction remains unproven.