Paleostress analysis of the Nyasa / Malawi Rift: implication for the
present-day regional dynamics
Abstract
The Nyasa/ Malawi rift is characterized by poor magma with relatively
large earthquakes. There has been a controversy as to the stress
kinematics of the rift, some considering it as part of the transform
fault and some considering it as a rift structure characterized by
normal faulting. To review this controversy, we collect fault slip data
from the central to the southern end of the rift and integrate our
results with published focal mechanisms fault slip data on the rift.
Results show that the central part of the rift is under radial extension
whereas the southern half is under oblique NNE-SSW transtensive tectonic
regime with the horizontal axis of minimum extension = 020˚. Further
south, the obliquity extension rotates by about 15˚ reaching N-S with
Shmin = 175˚. The level of structural penetration and intensity of
faulting show that the N-S opening is more important and prominent in
the south than towards the north. We also find that the faults that dip
to the east and trending NW-SE are characterized by sinistral sense of
movement whereas those that dip to the southwestern side are
characterized by dextral sense of movements. This implies that
regionally, the rift is essentially under normal faulting regime but
with a significant strike –slip component – hence the obliquity
kinematics. Tectonic regimes obtained from fault-slip data are related
to lithospheric scale and involve both the crust and the upper mantle.
Thus, the pure NNW-SSE extension related to focal mechanism data are
crust deformation related events.