World Stress Map Conference - International Lithosphere Program ...

3 rd World Stress Map Conference, 15.-17. October 2008 in Potsdam
Thursday, October 16 th
Poster Session VI: Stress field and interpretation at regional scale
Present-day stress field along the East-African Rift System
and implications on the rift dynamics
Damien Delvaux
Dept of Geology and Mineralogy, Royal Museum for Central Africa, Tervuren, Belgium
damien.delvaux@africamuseum.be
Andreas Barth
Geophysical Institute, Universität Karlsruhe (TH), Karlsruhe, Germany
The kinematic models and the associated orientation of extensional stress of the East-African
Rift System have been subjected to much debate since a long time. In the past decades the
proposed models relied on the interpretation of the overall rift geometry, geological fault-slip
data and the few focal mechanisms available. As a consequence of the lack of focal
mechanism data, these models generally suffer of a poor time control and an underestimation
of the possible changes in the stress field and geodynamic regime with time and space. In the
recent years, there has been a significant increase in the number of focal mechanisms
available for the entire rift system, and it is now possible to estimate the present-day stress
field in relative detail based on seismotectonic data alone. The first order stress field can be
determined with confidence for the whole East-African Rift and in some regions, the data
allow to access the second-order stress field. We compile available data from the
Global/Harvard CMT catalogue and various other sources and group them in a series of
distinct regions (boxes). The selection is based on the knowledge on the tectonic structure,
that allows us to define regions with the same dominating stress field. In a first step, large
regions are considered to get the overall stress pattern, and in a second step, where possible,
some large regions were split into smaller distinct ones for more details. For each box and for
the same data set, the average reduced stress tensor has been obtained by using both the
TENSOR program (Delvaux & Sperner, 2003) and the Slick method (Michael, 1984/1987).
The obtained stress pattern reflects a complex interaction between different driving forces, the
influence of rift structures and the large-scale topography. Some regions show a rapid lateral
change in stress regime and stress axis orientation. Within the rift system, there is a general
tendency for normal faulting stress regimes with a direction of extension orthogonal to the
trend of the major rift depressions. This is particularly well marked for the western rift branch
that surrounds the Tanzanian Craton, with directions of extension ranging from NW-SE to E-
W and NE-SW. In some restricted regions of the rift, the stress field might differ significantly
from this general trend, with a rapid change from the adjacent regions (both in horizontal
stress axis orientation and in stress regime). Within the topographic bulge that characterises
Zambia and neighbouring regions, a normal faulting regime dominates with a NW-SE
direction of extension, orthogonal to the general axis of the first order topographic high. This
might also be the main reason for the E-W extension along the Indian Ocean coast and in the
Mozambique Channel. In the regions of low topography outside the rift system and for which
data are available, stress regimes with E-W oriented SHmax direction dominate. In South
Sudan, the stress regime is intermediate between extensional and strike-slip, while the limited
number of data available for the Congo basin suggests that the region comprised between the
Atlantic Ocean and the western rift branch is presently affected by a compressional regime.
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