Chronology of volcanism and rift basin propagation: Rungwe Volcanic Province, East Africa

Abstract

Continental rifts are segmented along their lengths into 50 to 100 km‐long extensional basins, suggesting a genetic relationship with regularly segmented oceanic rifts. To investigate the spatial and temporal development of along‐axis segmentation in youthful continental rifts, field, remote sensing, and K‐Ar geochronology studies were conducted in four Western rift (East Africa) basins. Volcanism within the Rungwe region began in late Miocene time prior to or concurrent with the development of high‐angle border faults bounding the then isolated Karonga and Rukwa basins. The Usangu and Songwe border fault segments located between the Rukwa and Karonga basins developed in mid‐Pliocene and Pleistocene time, respectively. Within the northern Karonga basin, approximately 2–4 km of WSW‐ENE directed “thick‐skinned” extension is estimated both from extrapolation of late Pleistocene slip rates to 5 Ma and from surface fault geometries. Differential strains between these extensional basins of different ages are transmitted to adjoining segments by NW and ENE striking oblique‐slip transfer faults within comparatively high‐strain accommodation zones. Throws along transfer faults also accommodate 2 km (minimum) variations in depth to basement beneath adjacent basins, whereas monoclines and ramps accommodate differential uplift along the flanks of basins. The observed geometry of en echelon border fault segments linked by shorter oblique‐slip transfer faults is similar to patterns predicted in numerical models of en echelon normal fault interactions [e.g., Aydin and Pollard, 1982]. Eruptive centers for alkali olivine basalt and phonolite flows within the Rungwe region coincide with the tips of en echelon border fault segments and with high‐angle transfer faults linking discrete border fault segments. The locations of eruptive centers appear to be controlled by faults, and these centers generally have propagated northward along transfer faults during Plio‐Pleistocene time. Historic eruptions near late Miocene centers located at the southern end of the transfer fault system, however, argue against a northward migration of the magma source. Thus, an along‐axis propagation of “thick‐skinned” lithospheric extension links once isolated basins and produces an along‐axis segmentation of this youthful rift. Propagation of magmatism along transfer faults linking en echelon border faults locally contributes to the regular segmentation.