Evidence of recent faulting in the Rukwa rift (West Tanzania) based on radar interferometric DEMs

Abstract

This paper presents the results of a study where remote sensing products are combined in a GIS with cartographic and field information to provide evidence of Quaternary faults in the Rukwa Basin. Topographic models computed from map digitization were found to be of limited use in low relief areas, where active faulting is associated with subtle topographic steps. The radar interferometry technique with ERS data was used to generate very detailed digital elevation models which revealed topographic lineaments that were so far undetected. The analyses show however that some of them, once their topographic signature is detected, can be retrieved and interpreted in optical satellite imagery (LANDSAT, ASTER) and topographical maps. Although most of the data are extracted from the interferometric models, we suggest that the complimentarity between the various layers of information allows for improved field investigations. A comparison with SRTM DEM is also provided. Two main populations of lineaments are visible: (a) sharp lineaments striking NNW–SSE and (b) subtle features striking WNW–ESE; both are oblique to the main N315° rift direction. In the first category we find a prominent structure that has motivated a detailed field investigation and was found to correspond to a recent normal fault affecting the uppermost Holocene palaeo-shoreline of Lake Rukwa (Ilyandi sandy ridge). In the southern part of the ridge, the fault produced a clearly visible scarp whereas in the northern part it controls the drainage pattern of the Katuma River and is responsible for the formation of Lake Katavi. Based on geomorphologic evidence and trench analysis, the Katavi fault can be considered as a possible site for the aftershocks of the major earthquake sequence that occurred on 13th January 1910. The second category of lineaments involves a succession of large amplitude features that form “wavy structures” in the north-western part of the basin. Field measurements have confirmed that they were due to atmospheric perturbations in the only available radar interferometric pair. But based on the study of the GIS layers, some lineaments of that category really exist and are probably involved in tectonic processes. In the region of Ivuna, in the south-eastern part of the basin, we found that the important hydrothermal activity corresponds to the intersection of two directions of lineaments. It is also in that region that we found important fissure fields that were, according to local people, opened after an earthquake in 1996.