Rift localization in suture-thickened crust: Evidence from Bouguer gravity anomalies in northeastern Tanzania, East Africa

The tectonic framework of northeastern Tanzania consists of three major elements, the Archean Tanzania Craton, the Proterozoic Mozambique Belt, and a portion of the Eastern arm of the Cenozoic East African rift system, which developed within the Mozambique Belt adjacent to the craton margin. We present some 1500 new point gravity measurements from northeastern Tanzania that, when combined with existing gravity data, characterize further the gravity field over the craton-mobile belt suture, as well as over the rift structures. The primary new observation which can be drawn from our data compilation and analysis is that a short wavelength gravity low exists over the craton-mobile belt suture. In a previous study, it was shown that a gravity low also exists over the Tanzania Craton-Mozambique Belt suture to the north in Kenya, and thus there appears to be a consistent gravity low over this suture from at least 6°S to the northern termination of the craton at ∼3°N. Based on the observation that gravity lows over Precambrian sutures elsewhere commonly arise from thickened crust, the gravity low over the Tanzania Craton-Mozambique Belt suture can be attributed to a few (2-5) kilometers of crustal thickening within an area 100-200 km wide, an interpretation that is also supported by seismic studies of crustal structure in Kenya. The development of Cenozoic rift structures within proximity to suture-thickened crust in northeastern Tanzania and Kenya suggests that rifting in the Eastern arm of the East African rift system may have been localized by the presence of the suture-thickened crust.