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Research Institute for African Geology, Department of Earth Sciences, The University, Leeds 2
THE TERM deep-structure is very subjective, since no precise ‘Mason-Dixon Line’ can be drawn between deep and shallow structural environments. The wide-spread involvement of infrastructural basement, so often invoked as a criterion of depth, pre-supposes that all orogenic belts were occupied by thick sequences of geosynclinal sediments, with or without volcanic rocks. Whilst there is strong evidence in favour of this supposition, there are many older orogenic belts in Africa in which the geosynclinal phase, which usually immediately precedes orogeny, is absent; it is the purpose of this contribution to examine some of the features of those belts.
Radiometric dating of the pre-Silurian terrains of Africa has resulted in the recognition of at least five major events of orogenic importance as follows: (i) 550 ± 100 m.y. (Damaran-Katangan or Pan-African orogeny); (ii) 1100 ± 200 m.y. (Kibaran orogeny); (iii) 1850 ± 250 m.y. (including the Eburnian orogeny); (iv) 2500–2800 m.y. (Shamvaian orogeny); and (v) c. 3000 m.y. (un-named). Of particular significance in unravelling the internal features of belts affected by events (i)–(iii) is the fact that the continent was subjected to lengthy periods of Pre-Cambrian erosion and that, except for parts of West and North Africa, it has also remained largely continental since Lower Palaeozoic times; as a result, a number of structural levels in Pre-Cambrian-to-early Palaeozoic orogenic zones are exposed. In particular, however, orogenesis may be recorded in at least two important ways: (a) a geosynclinal facet of deformed cover rocks; and (b) a vestigial facet (vestigeosyncline) of rejuvenated floor-rocks (basement
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