Evolution of taphrogenic lineaments in continental platforms

Abstract

The Rift System of eastern Africa is taken as a basis for an enquiry into the nature and age of taphrogenic lineaments in continental platforms. Special emphasis is placed on those segments in which the early Precambrian infrastructure has been exposed by continued uplift and deep erosion, and the evidence points to a genetic association of the Neogene graben faults with dislocation zones originating in the Precambrian and affecting the deep lithosphere. These proto-rift dislocations were repeatedly rejuvenated during successive tectonothermal cycles, showing blasto-mylonites, flaser gneisses and migmatites according to tectonic depth, and culminating in brittle surface faulting. A fundamental difference between taphrogenic and orogenic processes is emphasized by the oblique crossing of younger fold belts by the lineaments. Geophysical and other evidence from Africa confirms data from the Rhinegraben and Lake Baikal showing that continental rift systems are underlain by zones of high heat flow and intrusions of anomalous low density mantle rock (penetrative convection), producing structures comparable with those beneath mid-ocean ridges. Geological mapping, however, proves that dilation is minimal and that rift volcanism is of continental type and, with small exceptions, is limited to a sector extending south from the Afar triangle in Ethiopia through Kenya. It is suggested that sea-floor spreading in the African rifts was prevented by their geotectonic position, and that the continent was under pressure from the active mid-ocean ridges surrounding it on three sides. Evidence is described from East Africa, and from the West African and Guiana Shields, indicating that continental lineaments extended into the ancestral rifts of sea-floor spreading. It is concluded that in the lithosphere, above the mantle lowvelocity zone, deep criss-crossing lineaments of ancient origin exist, of which some served in Precambrian times as dislocation zones and channels for penetrative convection and were thus pre-conditioned for periodic tectonothermal rejuvenation. Certain segments of these channels have been selected by the powerful rising mantle dorsals associated with sea-floor spreading, and thus play a part in plate tectonics.