Short term development of intracontinental rifts, with reference to the late Quaternary of the Rukwa Rift (East African Rift System)

Relatively low-resolution seismic data and hi,oh contemporaneous rift topog raphy normally limit quantitative analysis of normal faults in rifts. The a vailability of a recently collected high-resolution reflection seismic surv ey in the SE part of the presently active Rukwa Rift (East African Rift Sys tem) coupled with high sedimentation rates in the submerged part of the rif t makes detailed quantitative analysis possible. High-resolution (down to a bout 1 m) seismic data penetrate ca. 300 m (representing about 150,000 year s) of the uppermost sediments and show fine details of normal faulting and related structures. Displacements on the faults, which occur in the hanging wall of the major rift boundary fault, range from a few metres up to 100 m . They show increasing displacements with depth and characteristic rollover folding of the hanging walls. Fault propagation occurred upwards through t he rapidly accumulating lake sediments and was preceded by flexuring and fo lding of the sediments. The geometry of the seismic sequences with alternat ing wedge-shaped and tabular units reflects pulsed activity on the faults. Fault displacement rates of up to 1.6 mm yr(-1) have been estimated. Each e xtensional pulse has a duration of 1000s to 10,000s of years. The quiescent stages have a similar duration. Up to 2.5% extension has occurred during t he recent evolution. (C) 2001 Elsevier Science Ltd. All rights reserved.