<p>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.</p> <p>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. </p> <p>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.</p>

<p>The Victoria microplate is generally assumed to be internally rigid, i.e. non-deforming.  Here, we describe geomorphological evidence for active fragmentation of the microplate along the E-W to NE-SW striking Isuria-Utimbara fault system, Lake Victoria, in the Kenya-Tanzania transboundary region.</p> <p>The Isuria-Utimbara fault system has received little previous attention and is not recognised as seismically active. The fault system marks the northern boundary of the Mara River Basin and lies within the mapped extent of the Victoria microplate, an apparently relatively rigid block situated on the Tanzanian craton. The area is defined by low seismicity within the temporal limits of the instrumental record: seismicity is concentrated along the western arm (as well as, to a lesser extent, the southernmost part of the eastern arm) of the East African Rift (EAR). Here, we describe geomorphological evidence for geologically recent earthquake activity, which has produced scarps and alluvial fans in the hanging walls of the major escarpments. The scarps appear to be segmented, with typical segment lengths of approximately 15 km, and together sum to an along-strike length of approximately 100 km. The height of the scarps exceeds 8 m with a maximum height of 25 m (measured using TanDEM-X Digital Elevation Model (DEM) Global data which has a horizontal resolution of 12 m and an ~2 m height error). Considering the length of a typical segment, scaling relationships suggest the possibility for multiple >M<sub>w</sub> 6 earthquakes. If the segments slipped together, this would result in a maximum earthquake magnitude of >7. Although dating has not yet been carried out, a constraint on slip rate comes from displaced Neogene volcanics found above and below the main escarpment, which give a long-term vertical displacement rate of approximately 0.1mm/yr, comparable with stable continental intraplate settings. Our findings have implications for the seismic hazard of the region: although parts of the Mara River Basin are protected areas of great ecological importance, population density is increasing along the shores of Lake Victoria and a major gold mine lies directly to the south of the fault system. This fault appears to be fragmenting the Tanzanian craton, albeit at relatively slow rates, and cratonic settings are in general capable of producing large and damaging earthquakes due to the possibility for a large seismogenic thickness.</p>

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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 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 Ma old) on either side of the faults. If the age is correct, slip rates can be estimated to be on the order of 0.1mm 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, Clay Mineral Ratio (CMR) and 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.

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