Drainage incision, tectonic uplift, magmatic activity, and paleo-environmental changes in the Kenya Rift, East African Rift System: A morpho-tectonic analysis

Abstract

We examined average drainage incision in the northern, central, and southern Kenya Rift (NKR, CKR, and SKR) of the East African Rift System (EARS) in relation to tectonic uplift, magmatic activity, and paleo-environmental changes by: (1) Mapping Eocene to Holocene volcanic units using published geological and geochronological data; (2) Calculating the average incision rate (AIR) on the rift flanks by obtaining the difference between paleo-datum and modern topography and dividing the resulting regional incision by the age of volcanic units; (3) Calculating the total amount of incision (TAI) on the rift flank from stream longitudinal profiles using the normalized steepness index (Ksn) and concavity (θ); and (4) correlating the spatial and temporal drainage incision patterns with tectonic, magmatic, and climatic events. Our analysis showed higher AIR (maximum of ~250 mm/kyr) and higher TAI (maximum of ~1200 m) in the CKR after ~4.5 Ma. Pliocene increase in the AIR and TAI is also observed in other parts of the EARS and this has been attributed to increase in tectonic uplift rate. We found that the spatial and temporal patterns of the tectonic uplift inferred from the drainage incision to be well-correlated with known phases of magmatic activity. Moreover, we found that the increase in the AIR and TAI after ~4.5 Ma to be well-correlated with changes in vegetation from woodland to grassland. This suggests that the tectonic uplift coupled with magmatic activity are probably caused by major shifts in ecosystem in the study area.