Tectonic controls on the ecosystem of the Mara River basin, East Africa, from geomorphological and spectral index analysis

Abstract

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.

Contrasting Effects of Tectonic Faults on Vegetation Growth along the Elevation Gradient in Tectonically Active Mountains

“…Meanwhile, faulting can result in the rapid incision of valleys and the erosion of hillslopes. These changes not only alter local humidity and thermal conditions but also give rise to "catenas," which are sequences of soil variations extending from hilltop to middle slope and the valley bottom [17,57]. Vegetation experiences differences in insulation, wind shear, and local evapotranspiration between hilltops and valleys, which could affect vegetation growth [58].…”

“…For example, a local-scale study in a semi-arid fault-controlled river basin in Africa (~1500 m asl.) indicated that active normal faulting is an important factor in stabilizing vegetation growth patterns, which creates favorable hydrological and pedological conditions along the escarpment [17]. Tectonic basin and associated alluvial fan (~1000 m asl.)…”

“…Geologic tectonism, a product of Earth's internal movements that determines the distribution pattern of geomorphology and mountain building/folding, strongly influences the altitudinal zonation of soils and biota [6,17,18]. It is broadly understood that tectonic forces influence the pace and pattern of landscape evolution by controlling landscape relief and the physical and chemical processes that move sediment and dissolve bedrock, respectively [19].…”

“…It is broadly understood that tectonic forces influence the pace and pattern of landscape evolution by controlling landscape relief and the physical and chemical processes that move sediment and dissolve bedrock, respectively [19]. Recent studies have revealed both beneficiary and disadvantageous effects of tectonic activity on ecosystems, particularly in tectonically active landscape [17,20]. For example, rock fracturing resulting from tectonic faulting determines the distribution of plant-available water and nutrients stored in soils, thereby enhancing the local water holding capacity [21].…”

“…On the other hand, tectonism-related earthquakes and landslides are major sources of ecological disturbances, which affect vegetation structures and dynamics [22]. Notably, the effects of tectonic activity on the heterogeneity of the local ecosystem in a fault-controlled river basin (also known as a tectonic basin) have been well illustrated [17,23]. However, the limited understanding of how tectonic processes influence vegetation growth along the elevation gradient in tectonically active mountains is a significant gap.…”