Morphostructure, paleostress and kinematics of the southern kilombero rift basin, southern Tanzania

Abstract

In recent years, there has been exploration for hydrocarbons in the Kilombero rift basin. However, detailed information regarding the structural pattern and kinematics within the basin, as well as the border fault has not been established. Structures play a crucial role in determining the presence of gas/and oil in a reservoir, making them vital in assessing hydrocarbon resources. Specifically, understanding whether the basin, its border fault (s) and intrabasinal faults are seismically active or not is essential in evaluating the potential for hydrocarbon reserves in a given area. This study therefore, focuses on conducting a morphostructural and paleostress analysis of the southern Kilombero rift to establish important fault parameters, including the current rift kinematics. In the morphostructural analysis, we find that the southern Kilombero rift border fault is a nearly 100 km long segmented structure characterized by an escarpment height of approximately 1300 m. Additionally, analysis of about 40 fault/extensional fracture surfaces indicates that the border fault, which is believed to be from the Early Pliocene period (around 5 ± 1 million years ago) and trending about 042°/222° (NE-SW), is under pure extensional stresses characterized by horizontal minimum principal stress axis (Shmin) of about 060°, implying that faults trending 330°/150° (NW-SE) are the ones associated with this extensional direction. However, a computed stress tensor from focal mechanisms show Shmin = 112° (ESE-WNW) meaning that NNE-SSW trending faults (i.e. 022°/202°) are currently active. Presence of these two pure extensional regime (with minor compression component) imply that the stress regime has either rotated through 52° in a clockwise direction from Shmin = 60° to Shmin = 112° in the Quaternary or that this latter extensional direction computed from seismic data represents a local/temporary tectonic phenomenon caused by changing far-field stresses. The stress tensors derived from our modeling efforts shed light on the regional stress conditions within the southern segment of the East African Rift System (EARS), to which our study area belongs. These revelations collectively suggest a transitional evolution of the geomorphology within the southern Kilombero rift basin, with a shift from a half graben configuration towards a full graben-horst-full graben system. This structural transformation enhances the basin's potential as a viable candidate for hydrocarbon accumulation. Strike-slip faults are also present but are not significant.