TECTONIC CONTROLS ON RIFT BASIN MORPHOLOGY - EVOLUTION OF THE NORTHERN MALAWI (NYASA) RIFT

Radiometric (K-Ar and Ar-40/Ar-39) age determinations of volcanic and volcaniclastic rocks, combined with structural, gravity, and seismic r eflection data, are used to constrain the age of sedimentary strata co ntained within the seismically and volcanically active northern Malawi (Nyasa) rift and to characterize changes in basin and flank morpholog ies with time. Faulting and volcanism within the Tukuyu-Karonga basin began at approximately 8.6 Ma, when sediments were deposited in a broa d, initially asymmetric lake basin bounded on its northeastern side by a border fault system with minor topographic relief. Extension, prima rily by slip along the border fault, and subsequent regional isostatic compensation led to the development of a 5-km-deep basin bounded by b road uplifted flanks. Along the low-relief basin margin opposite the b order fault, younger stratigraphic sequences commonly onlap older wedg e-shaped sequences, although their internal geometry is often prograda tional. Intrabasinal faulting, flank uplift, and basaltic and felsic v olcanism from centers at the northern end of the basin became more imp ortant at approximately 2.5 Ma when cross-rift transfer faults develop ed to link the Tukuyu-Karonga basin to the Rukwa basin. Local uplift a nd volcanic construction at the northern end of the basin led to a sou theastward shift in the basin's depocenter. Sequence boundaries are co mmonly erosional along this low-relief (hanging wall) margin and confo rmable in the deep lake basin. The geometry of stratigraphic sequences and the distribution of the erosion indicate that horizontal and vert ical crustal movements both across and along the length of the rift ba sin led to changes in levels of the lake, irrespective of paleoclimati c fluctuations.