1. Project PROBE, Department of Geology, 310 Old Chem, Duke University, Durham, North Carolina 27706, USA
2. ^*Present address: Texaco, Latin America/West Africa, 150 Alhambra Circle, PO Box 343300, Coral Cables, Florida 33134, USA.

Abstract

Continental rift systems are rips in plates caused by focusing of extensional stresses along some zone. In the same way that tensile cracks in the side of a brick building generally follow the mortar between bricks, rifts initially follow the weakest pathways in the pre-rift materials. There has even been a suggestion that the occurrence of rifts is controlled by pre-rift structure^1–3, although many workers do not subscribe to such direct causality. The relationships between pre-rift structure and rifting tend to become progressively more obscure as the scale of examination decreases, mainly because unravelling these relationships requires more pre-cise information about the architecture of rifts, the nature of the pre-rift fabric and about the principal axes of stress than is generally available. Recent work by Project PROBE on the Tanganyika^4–15 and Malawi Rift Zones^12,16–22 permits crucial aspects of these inter-relationships to be examined in a more rigorous fashion than was previously possible. In this region, the rift system follows Proterozoic mobile belts and bifurcates around the Tanganyika craton, which apparently has acted as a resistant core (Fig. 1). Here we show that the fundamental architecture of the Tanganyika and Malawi Rift Zones (that is, the positioning of half-grabens, the way they link together, and the types and trends of linking structures) is strongly influenced by the nature of the pre-rift fabric and the orientation of the stress field.