Chapter 5 Tectonic development of the rift structures

During the Neogene–Recent there have been two major phases of crustal deformation in northern Tanzania. Each deformation event has been followed by a major pulse of volcanism arising from instabilities in the mantle. The chronology of the development of the rift faulting has been made possible by the dating of volcanic rocks that erupted before and after particular episodes of faulting. Appendix 1 (Dawson 2008) lists the current dating results for the volcanic rocks. However, most of the dates were obtained many years ago by the K–Ar method and more refined stratigraphy will doubtless arise from Ar/Ar dating in the future. Whereas in Kenya, there is evidence for an elongate pre-rift depression in which the earliest volcanics were deposited (Baker 1986), there is at present only sparse evidence for the presence of pre-volcanic sedimentary basins in northern Tanzania. The limited evidence comes from an exposure at the base of the rift escarpment at the north end of Lake Manyara, where a sedimentary formation, termed the Manyara Group, comprises a boulder conglomerate unconformably overlying metamorphic basement rocks. The conglomerate itself is overlain by siltstones, sandstones, waterlain tuffs and ashes with an intercalated basalt that has been dated at 4.86 ± 0.24 Ma. This is taken to indicate that, in this area, limited basin subsidence (? related to faulting) and basaltic volcanism had begun by around 4.9 Ma (Foster 1997; Foster et al . 1997). This shallow-basin formation follows the oldest volcanism in the area, the eruption of Essimingor lying 25 km to the east which has rocks dated between 8.1 and 3.2 Ma. In some respects this is analogous to western Kenya, where Miocene basin formation was preceded by the eruption of highly explosive volcanoes such as Mt Elgon and Napak (Baker et al. 1972). The main evidence for …