North-South variation in tectonic activity along left-stepping extensional basins revealed by morphometric analysis: Gofa province, southwestern Ethiopia, East Africa

We report on the morphotectonic characteristics in the tectonically active Southern Ethiopia Rift (SER) based on the analysis of high-resolution topographic data (12m TanDemX) and satellite imagery. The study region is a wide zone of distributed extension at the transition from the SER and the Northern Kenyan Rift and reflects the long-term effects of episodic tectonic events in the landscape.  The uplifted footwall margins of the north-south trending and left stepping ēn ēchelon basins of the SER constitute Pan-African basement rocks in the southern and central part (Chew Bahir, Mali-Dancha and part of Beto) and tectonized Miocene basalts in the north (Sawula). As such this region is an ideal location to record the tectonic characteristics of a major transition zone between two rift systems. Some of the unsolved problems in this area concern the degree of tectonic activity, spatiotemporal variations in the amount of extension, and the nature of kinematic linkage between different faults. To examine these issues, we calculated morphometric indices of river catchments along major fault-bounded blocks as proxies for tectonic activity and combined this information with structural, seismicity, and climatic data. 
We determined basin asymmetry, hypsometric integral, mountain-front sinuosity, valley floor to valley-width-height ratio, basin shape, the range of basin form and mean slope; additionally, we calculated knickpoint distributions and channel-steepness index values from 89 sub-basins. Combined, the data suggest a significant north-south variation in extensional processes. For example, in the northern basins knickpoints are generally located in upstream areas near the channel heads. They are rare in the Mali-Dancha basin, whereas in the Chew Bahir basin a distinct distribution along the main channel is recognized from basin head to the mountain front. In the south the knickpoints are closest to the mountain front. This unique spatial arrangement of knickpoints in rivers draining the footwalls of extensional blocks in the north-south transect suggests a gradual, southward-directed shift in extensional deformation and recent tectonic activity. The normalized channel-steepness index value is generally small; however, it also exhibits a significant southward trend with higher values (i.e., tectonic activity). Additionally, the normalized channel steepness indices are higher at orthogonally interacting faults compared to neighbouring areas, suggesting strain localization. 
Our new results suggest a northward increase in the geomorphic maturity of the analyzed sub-basins from Chew Bahir (juvenile) to Sawula (mature), which is compatible with a northward decrease in tectonic activity and a dominance of erosional processes. This is consistent with published, northward-decreasing extension rates and the degree of regional seismicity. Furthermore, strain localization at interacting faults suggests kinematic linkage of the left-stepping bounding faults of the sub-basins.