ARC ASSEMBLY AND CONTINENTAL COLLISION IN THE NEOPROTEROZOIC EAST-AFRICAN OROGEN - IMPLICATIONS FOR THE CONSOLIDATION OF GONDWANALAND

Understanding of the style and timing of collision between East and West Gondwanaland to form the East African Orogen is incomplete, but the general outline of this event is slowly emerging. A model embodying the present understanding of this evolution is presented. Present reconstructions of the earliest stages in the EAO orogenic cycle may be more speculation than understanding, but initiation by rifting of a continent is consistent with the data. This rifting is identified as part of the break up of Rodinia at about 850-900 Ma. The sequence of events that begins with sea-floor spreading and formation of arcs and back-arc basins and continues with the accretion of these tectonic cells into juvenile crust is coming into sharper focus, at least for the Arabian-Nubian Shield. There is evidence that these processes were underway as early as 870 Ca and continued until at least 690 Ma. Continental collision first led to crustal thickening and uplift, beginning perhaps as early as 750 Ma but certainly by 700 Ma, and continued with orogenic collapse and escape tectonics until the end of the Precambrian. At least as far as the EAO is concerned, this dates the consolidation of East and West Gondwanaland. Tectonic escape led to the development of major rift basins in the northern EAO and environs which led directly to sea-floor spreading and formation of a passive margin on the remnants of Gondwanaland and the formation of an ocean basin to the north, at about 550 Ma ago. -from Author