ACTIVE FLUVIAL REORGANIZATION OF AN EARLY-STAGE RIFT: INSIGHT FROM THE NEOTECTONICS OF THE ALBERTINE RIFT BASIN, EAST AFRICA

The Northern Western Branch of the East African Rift System (EARS) serves as an ideal natural laboratory to investigate the influence of early-stage continental rifting on the reorganization of fluvial networks. The Albertine rift basin, NW branch of the EARS, exhibits a full graben geometry that is ~60 km wide, bounded by dominantly NE-striking large rift faults with uplifted rift shoulders. The Lake Albert rift basin is bounded to the southeast by the 90 km-long predominantly dip-slip Tonya Fault and to the northeast by the 130 km-long dip-slip and apparent sinistral Toro Bunyoro Fault that changes in slip magnitude along strike. We utilized 30-m-resolution SRTM Digital Elevation Models (DEMs) and field observations to characterize fluvial channel morphologies adjacent to and flowing into the Albertine Rift Basin. Topographic swath profiles and channel steepness (Ksn) across rift bounding faults help to better understand the influence of tectonic uplift on fluvial network reorganization and modification of the present-day landscape. Swath profiles show variations in vertical fault separations along the Tonya-Toro Bunyoro Fault system. The Tonya Fault exhibits a minimum vertical separation of 475 (± 100) m that decreases to 65 (± 35) m along the Northern Toro Bunyoro Fault, where observations are consistent with a sinistral offset fluvial channel. Distinct segments of the fault show higher Ksn values in the vicinity of steep border fault scarps, interpreted to indicate zones of active faulting and show that major fluvial drainage channels crosscutting the border fault are in a transient state. The results demonstrate how along-fault variation of slip modulates spatial patterns of stream reorganization during early-stage rifting.