PERSPECTIVES ON THE EFFECT OF PRE-EXISTING WEAKNESSES IN THE MALAWI RIFT, EAST AFRICA

The Malawi rift is localised within rocks of Precambrian mobile belts, and comprises a series of asymmetric border faults with lengths of up to 100 km. On the rift valley scale the border faults have been suggested, at least to the first order, to exploit pre-existing weaknesses in the ancient mobile belts. It is, however, unclear what these weaknesses are, and at what scale and depth range they may be reactivated.

We know from exhumed analogues of lower crustal rocks that viscous deformation may localize within weak precursors defined by fine grain size or well-oriented crystal preferred orientations, but it is unclear whether such structures will also be weak in the brittle crust. A question of whether frictional reactivation is likely arises from both metamorphic petrology and rock mechanics. The metamorphic history of Malawi rocks involve burial and heating to at least amphibolite facies in up to three orogenic events, and associated dehydration of the rocks such that local fluids as well as weak hydrous phases are absent in most or all the basement terrane rocks. Rock deformation experiments confirm a lack of frictionally weak materials.

To test the fault-scale effect of pre-existing weaknesses on rift evolution we have studied the ~130 km long Bilila-Mtakataka fault in detail, as an example of a major border fault in southern Malawi, away from known magmatic centres. We find that although this fault locally exploits metamorphic foliations, this only occurs if the foliations are well oriented for reactivation. Otherwise, the fault follows an orientation that is neither consistent with existing fabrics nor with current rift kinematics. On the other hand, the orientation of the surface scarp is well fitted with upward propagation of a planar structure extending from a depth of ~ 10 km, and that was well oriented for reactivation or nucleation in an earlier phase of rifting.