EVIDENCE FOR PULSES OF SHORT-TERM (103-104 YR) ACTIVITY ON ACTIVE NORMAL FAULTS IN THE RUKWA RIFT

A high-resolution reflection seismic survey in the SE part of the presently active Rukwa Rift (East African Rift System), shot in an area of high sedimentation rates in the submerged part of the rift, has made possible quantitative analysis of the recent faulting history. The seismic data reveal fine details of the uppermost ca. 300 m (representing the last ca. 150,000 yr) of the sediments that are cut by normal faults. Visible displacements on the faults range from a few meters up to ca. 100 m and generally increase with depth. Most faults reach close to the lake floor and some can be shown to produce small scarps less than 1 m high. Other faults are blind and do not reach the sediment surface. The hanging walls of the faults show rollover folding at depth with fault-propagation folds formed above blind faults. The geometry of the seismic stratigraphically-defined sequences shows alternating wedge-shaped and parallel units that reflect pulsed activity on the faults. Based on an extrapolation of recent sedimentation rates, the extensional pulses seem to have durations of 10³-10⁴ yr.