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

Radiometric (K-Ar and 40Ar/39Ar) age determinations of volcanic and volcaniclastic rocks, combined with structural, gravity, and seismic reflection data, are used to constrain the age of sedimentary strata contained within the seismically and volcanically active northern Malawi (Nyasa) rift and to characterize changes in basin and flank morphologies with time. Faulting and volcanism within the Tukuyu-Karonga basin began at approximately 8.6 Ma, when sediments were deposited in a broad, initially asymmetric lake basin bounded on its northeastern side by a border fault system with minor topographic relief. Extension, primarily by slip along the border fault, and subsequent regional isostatic compensation led to the development of a 5-km-deep basin bounded by broad uplifted flanks. Along the low-relief basin margin opposite the border fault, younger stratigraphic sequences commonly onlap older wedge-shaped sequences, although their internal geometry is often progradational. Intrabasinal faulting, flank uplift, and basaltic and felsic volcanism from centers at the northern end of the basin became more important at approximately 2.5 Ma when cross-rift transfer faults developed to link the Tukuyu-Karonga basin to the Rukwa basin. Local uplift and volcanic construction at the northern end of the basin led to a southeastward shift in the basin's depocenter. Sequence boundaries are commonly erosional along this low-relief (hanging wall) margin and conformable in the deep lake basin. The geometry of stratigraphic sequences and the distribution of the erosion indicate that horizontal and vertical crustal movements both across and along the length of the rift basin led to changes in levels of the lake, irrespective of paleoclimatic fluctuations.