HR: 09:40h
AN: U51B-06    [Abstracts]
TI: Patterns of Stress and Strain Rate in Southern Africa
AU: Ben-Avraham, Z
EM: zvi@terra.tau.ac.il
AF: Department of Geological Sciences, University of Cape Town, Rondebosch, 7701 South Africa
AU: * Bird, P
EM: pbird@ess.ucla.edu
AF: Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AU: Schubert, G
EM: schubert@ucla.edu
AF: Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567 United States
AU: Andreoli, M
EM: marco@necsa.co.za
AF: South African Nuclear Energy Corporation, P.O. Box 582, Pretoria, 0001 South Africa
AU: Viola, G
EM: Giulio.Viola@NGU.NO
AF: Geological Survey of Norway, Leiv Eirikssons vei 39 (Lade), Trondheim, N 7491 Norway
AB: The southward propagation of the East Africa rift presents an opportunity to study incipient plate-boundary formation. We tabulate orientation data which confirm the province of NW-SE-directed most-compressive horizontal principal stress ("Wegener stress anomaly") earlier tentatively attributed to ridge-push. We use thin-shell finite-element models with realistic rheology to test three causes of stress: (1) lateral variations in density moment; (2) resistance of unbroken lithosphere to relative plate rotation; and (3) stress concentration ahead of a crack tip. Models in which stress is due primarily to variations in density moment are relatively unsuccessful in their predictions (59~73% incorrect regimes; 32~40° azimuth errors). Models in which Africa-Somalia spreading is regulated at realistic rates by remote boundary conditions are more accurate (18~41% incorrect regimes; 25~35° azimuth errors). Treating the East Africa rift as a frictionless crack degrades the fit in either case. This suggests that the "Wegener stress anomaly" is caused primarily by the relative rotation between the Somalia and Africa plates. The East Africa rift north of 21°S may be weakened by strain, but must have residual friction >0.1. The expected greater strength of oceanic lithosphere is likely to cause dramatic stress increases, reorientations, and regime changes offshore. The predicted strain rate map has high rates along the rift, curving at 12°S into a western arc through Angola-Namibia-South Africa. Seismic hazard in Namibia may be greater than the instrumental catalog suggests. However, a number of unfit data indicate that these models represent only a first step.
UR: http://element.ess.ucla.edu
DE: 8107 Continental neotectonics (8002)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 9305 Africa
SC: Union [U]
MN: Fall Meeting 2005