Tectonic controls on the ecosystem of the Mara River basin, East Africa, from geomorphological and spectral index analysis
Authors: Ludat Alina Lucia , Kübler Simon 
Published: May 2023

Abstract. Tectonic activity impacts the environment; therefore, identifying the
influence of active faulting on environmental factors, such as soil
development and vegetation growth patterns, is valuable in better
understanding ecosystem functions. Here, we illustrate how tectonic activity
and the lithology of bedrock influence temporal and spatial patterns of
vegetation and soil parameters in a fault-controlled river basin. The Mara River basin lies in a region of previously unrecognised active
normal faulting, dominated by the Utimbara and Isuria faults, resulting in
areas of relative uplift, subsidence and tilting. Faulting leads to
spatially variable erosion and soil formation rates as well as disruption
and modification of drainage systems. On a small scale, steep escarpments
cast shade and provide shelter. All of these factors might be expected to
exert controls on ecosystem dynamics on a range of lengths and
timescales. Here, we investigate tectonic controls on ecological processes
in the Mara River basin using TanDEM-X and Sentinel-2 data. We use
high-resolution digital elevation models (DEMs) to map the Utimbara and
Isuria faults and to measure the height of the escarpments (up to 400 m)
along the length of the faults. Total fault offset can be estimated by
correlating Neogene phonolite lavas (thought to be 3.5–4.5 Myr old) on
either side of the faults. If the age is correct, slip rates can be
estimated to be on the order of 0.1 mm yr−1. Analysis of DEMs also
reveals the presence of recent earthquake scarps in the hanging-wall
sediments of the main faults and extensive alluvial fan formation on the
hanging wall. Low mountain front sinuosity values and the presence of steep
escarpments also suggest recent activity. Drainage is displaced across the
fault traces, and, in one area, it is possible to map the lateral channel
migration of the Mara River due to hanging-wall tilting. We used a 5-year normalised difference vegetation index (NDVI) time series,
the clay mineral ratio (CMR) and a moisture stress index (MSI) to investigate
spatiotemporal vegetation patterns and soil formation. Whilst lithology does
exert some control, as expected, we observed 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,
likely caused by favourable hydrological and pedological conditions along
the escarpments; tectonic activity has a direct beneficial influence on
ecological processes in this climatically sensitive region.