INTEGRATION OF EAST-AFRICAN PALEOSTRESS AND PRESENT-DAY STRESS DATA - IMPLICATIONS FOR CONTINENTAL STRESS-FIELD DYNAMICS

Borehole breakout studies, aligned Quaternary volcanic vents, and kinematic analysis of Quaternary faults indicate that the present-day least horizontal stress direction (S(hmin)) in Kenya is aligned approximately NW-SE and in central Sudan is nearly N-S. Limited data in eastern Ethiopia suggest a NE-SW S(hmin) orientation. The regional pattern for S(hmin) is therefore roughly radially disposed about the Afar plate junction. Geologic structures in the Gregory Rift, western Kenya, suggest that up to 0.6-0.4 Ma, S(hmin) was oriented E-W, normal to the main rift trend. Similar arguments for the Ethiopian Rift indicate that during its main phases of extension, S(hmin) was oriented NW-SE. These data indicate that the central east African stress field underwent a significant realignment during the Quaternary, rotating about 45-degrees in a clockwise in the Kenyan Rift. Similar rotations may have occurred in eastern Sudan and eastern Ethiopia on the periphery of the Ethiopian Rift. In Kenya, this stress reorientation resulted in dextral oblique reactivation of older normal faults and formation of new normal faults with dextrally oblique slip components. The large-scale structural framework of the Kenyan rift is no longer suitably oriented to accommodate large extensional strains. East African extension may now be transferred to the western branch of the rift system (Tanganyika-Malawi rifts) and rejuvenated basins south of the Gregory Rift in Tanzania (Eyasi-Manyara rifts). Rapid rotations of near-surface stress tensors (of the order of 7.5-degrees/10(5) years in the case of East Africa) are now documented in several continental extensional systems. It is unlikely that large-scale mantle circulation patterns can fluctuate at such rapid rates. This suggests that drag at the base of the lithosphere, at least in continental extensional areas, is not the dominant force controlling the orientation of the near-surface stress field. Rather, intraplate forces, produced and propagated from distant plate boundaries (ridge-push, subducting-slab-pull), may play a greater role in configuring the continental crustal stress regime.