Architecture of the Lake Malawi Rift, East Africa

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Abstract Multichannel seismic data reveal that the Malawi Rift is composed of seven linked half-graben units. Master fault systems with dip-slip offsets of 4km or more border the sediment-filled basins. Most faults within the half-graben dip subparallel to the controlling border fault system, although they are not necessarily parallel in plan view. Half-graben alternate directions of asymmetry at intervals of 60 to 100 km along the axis of the rift, with border fault systems occurring predominantly on either the eastern or western margins. Distinctive structural expressions (termed accommodation zones) occur in areas between changes in the direction of half-graben asymmetry. These zones, which may be characterized by considerable oblique-slip faulting, are typically expressed as basement highs between the sub-basins they divide. The Malawi seismic data further show that a wide range of morphologic expressions are possible in regard to accommodation zones, ranging from complexly faulted antiformal highs to simple horsts to even topographic lows. These seismic data also show that accommodation zone morphology is not always specific to particular half-graben linking arrangements and that morphology can vary considerably along the strike of an individual accommodation zone; Accordingly, accommodation zones are categorized as interference, strike-slip, or isolation zones. The terms low-relief and high relief were used in previous PROBE publications. Measurements of reflector displacement on migrated seismic sections yield values of about 7% extension for the uppermost brittle crust. Extension across the rift is approximately east-west if it is assumed that the faults displaying the greatest amount of dip-slip offset are normal faults. The thickest sedimentary sections and greatest relief of flanking uplifts are found in the northern half-graben units. If the relative ages of the rift units can be correlated to the thickness of the sedimentary section and relief on the flanking uplifts, then the northern half-graben are older and rift ages to the north.