The influence of tectonic surface faulting on vegetation growth and soil formation of the Mara River Basin, East Africa

Tectonic activity impacts the environment and identifying the influence of active faulting on environmental factors, such as vegetation growth and soil formation patterns, is valuable in better understanding ecosystem functions. We applied remote sensing techniques to illustrate how tectonic activity and lithology of bedrock influence temporal and spatial patterns of vegetation and soil parameters in a climatically sensitive, fault-controlled river basin in the Kenya-Tanzania transboundary region.

The Mara River Basin lies in a region of previously unrecognised tectonic activity, characterised by subrecent extensional faulting along the Utimbara and Isuria faults. Faulting leads to spatially variable erosion and soil formation rates as well as disruption and modification of drainage systems. All these factors might be expected to exert controls on ecosystem dynamics on a range of lengths and timescales. We investigate tectonic controls on ecological processes in the Mara River Basin using a combination of geospatial mapping and multispectral image analysis. To map fault structures and to reveal signs of recent tectonic activity along the Utimbara and Isuria faults, we use high-resolution digital elevation models derived from 12m TanDEM-X data. To investigate spatiotemporal vegetation patterns and soil formation, we use a 5-year Normalised Difference Vegetation Index (NDVI) time-series, Clay Mineral Ratio (CMR) and Moisture Stress Index (MSI) derived from Sentinel 2 data. 

Whilst lithology does exert some control on ecological properties, we also observe that the downthrown hanging wall of the faults, especially directly adjacent to the escarpment, is consistently associated with a higher degree of vegetation, wetland formation and clay distribution. Analysis of spectral indices shows that the overall spatial pattern of vegetation cover is seasonally low in the flat plains and perennially high in the vicinity of more complex, tectonically influenced structures. The NDVI highlights several locations with permanently healthy vegetation along the escarpment which extend downslope for several kilometres. Our study shows that in the Mara River Basin, active normal faulting is an important stabiliser of vegetation growth patterns. We interpret this effect to be caused by favourable hydrological and pedological conditions along the escarpments and tectonically induced structures such as subrecent surface ruptures and a series of small, fault-bounded alluvial fans exposing systematically high vegetation and clay values. This implies that tectonic activity has a direct beneficial influence on ecological processes in this climatically sensitive region. As future climate change in the area is expected to lead to accelerated habitat desiccation and deterioration of vegetation quality, suitable habitats for wildlife will progressively reduce and will likely be limited to tectonically active locations. Long-term insights into tectonic processes and the interplay between geology and soils can thus be useful for recent and future ecosystem management since the understanding of an area from a geological perspective can complement the understanding of other natural processes within it.