Interpretation of refelction seismic data from the Usangu Basin, East African Rift System

Abstract

Reflection seismic data collected by AMOCO Tanzania Oil Company in the 80s, are herin used to correlate the main Usangu Basin parameters to those estimated from outcrop, gravity and aeromagnetic data. The seismic data are further used to constrain the geometry of crustal extension in this south-western sector of the East African Rift System (EARS) in Tanzania. Three distinct sedimentary packages on top of the basement reflector are interpreted and correlated to the Permo-Carboniferous Karoo rocks, the Jurassic/Cretaceous and Miocene Red Sandstone Group and Miocene to recent Lake Beds in the adjacent Rukwa basin, where the ages of the sedimentary sequences have been established on the basis of drill hole data.

The Karoo beds, deposited on an undulating weathered basement surface are relatively thin (~ 200m). The Red Sandstone Group reach a maximum thickness of up to 420 m while the Lake Beds are up to 289 m thick. Beneath the Karoo, seismic reflections show interval velocities of 3.22 km/s, which are less than typical crystalline basement velocities of 4.2 km/s in the adjacent Rukwa Rift. These “sub-basement” velocities are attributed to the slightly metamorphosed sediments of Upper Proterozoic, Buanji Group that crop out extensively on top of the Chimala scarp. Previous estimate of the thicknesses of Buanji sediments from outcrop data has been up to 1.086 km. The present study, however, suggests a thickness of more than 2 km for the Buanji sediments beneath the Mesozoic to recent sediments in the Usangu Basin. Age wise the Usangu Basin, which has always been considered to be much younger than its south-western continuation, the Luangwa valley, is here suggested to be an old structure in which sedimentation has been going on since Karoo time. Recent faulting mechanism inferred from recent earthquake data and the present interpretation of seismic sections suggest that deformation in the Usangu Basin has been predominantly strike slip. Thus, the Usangu Basin is best interpreted as a transfer fault, accommodating the relative movement between the southern-most sub-basin of the Rukwa Rift and the northernmost sub-basin of the Malawi Rift.