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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.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 >Mw 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.

Salt diapirs are prominent features in the Zagros fold-and-thrust belt displaying complex interplay between the buoyant forces driving the rock salt to the surface, distribution of the caprock and erosion. The caprock represents the solid residue of salt dissolution in the apical part of the diapir. Understanding the dynamics of subaerial spreading of salt in salt glaciers therefore requires knowledge about the spatiotemporal surface deformation and the underlying controlling factors. However, the contemporary vertical surface-deformation pattern across salt diapirs is difficult to detect and interpret along disciplinary boundaries. Therefore, our goal is to analyse the active surface-deformation patterns of diapir-caprock systems in the Zagros, where diapirs and their caprocks are well exposed and accessible for field mapping. We primarily integrate high-resolution Persistent Scatterer Interferometry (PSI) and field mapping. We used Persistent Scatterer Interferometry (PSI) to obtain the highest available spatiotemporal resolution (on the range of mm/yr, ~12-day repeat cycle) of the vertical surface-deformation pattern across the Karmostaj and Siah Taq salt diapirs for which previous knowledge from detailed geological field mapping is available. Both diapirs contain a thick and deformed caprock layer on top or in the surroundings of the salt cupolas and are located 20 km south from the Lar city in southern Iran. We analysed the PSI-data using Sentinel-1 images acquired between October 2014 and December 2018 using the German Aerospace Center’s (DLR) Integrated Wide Area Processor (IWAP). First, the time-series analysis of the deformation signal in the line-of-sight is investigated for seasonal effects correlations, such as precipitation and heat. Second, the line-of-sight signal is split into vertical and horizontal components. In the next step, geological observations from field mapping provide the context to interpret the geodetic data. Preliminary results indicate that the deformation identified from PSI signal decreases outwards from the apical part for salt diapirs with thick caprock, as the caprocks and other residuals get relatively thicker above the underlying viscous salt. We postulate that the extra load the caprock body exerts on the underlying ductile salt drives subsidence in the crestal portion of the diapir. Understanding the spatiotemporal deformation pattern helps to recognize the impact of dissolution/erosion mechanisms and the distribution of caprock on salt diapirs. Therefore, careful analysis of geodetic signals, which requires contextual integration with geological field observations, reveals the influence of caprock on salt movement.

In the emergent (subaerial) salt diapirs, the salt faces negative buoyancy when extruded to the surface, and flows outward around their vent by gravity spreading. It also faces dissolution and erosion. Salt supply, salt flow, dissolution, and erosion also influence the diapir’s shape. Although satellite geodesy monitors the surface deformation of the salt-caprock glacier system, the interpretation of the resulting deformation pattern in terms of salt supply, dissolution, and erosion is not straightforward. To overcome these shortcomings, we analyze surface deformation pattern of a fountain-shaped and nearly symmetrical diapir (Finu) within the Zagros Belt of Iran using Persistent Scatterer Interferometry (PSI). The PSI data are extracted from the Sentinel-1 SAR images using the Integrated Wide Area Processor (IWAP) at the German Aerospace Center (DLR) covering four years from October 2014 to December 2018. The line-of-sight signal from the PSI data is decomposed into the vertical and horizontal deformation signals. Within the diapir, the deformation signal is then spatially correlated with the influencing factors, including local position within the diapir, slope, karstification, and drainage. Along an E-W profile across the diapir, two-dimensional deformation vectors reflect salt supply and spreading; therefore, the magnitude and direction of these vectors are influenced by their local position within the diapir and the slope. There is a slight uplift in the central part of the salt domes with active salt extrusion. The deformation vectors divert outward in the slope direction, and the deformation reaches its maximum magnitude at the upper flanks of the central dome. The deformation decreases in the outer flat plateau regions of the extrusions and continues to decrease in the steep slopes at their lateral terminations. Along the same profile, relatively higher subsidence is detected in areas where sinkholes are abundant. In these regions, salt is removed in the subsurface by dissolution-driven karst development in contrast to areas where the surface drainage system is developed, and fluvial erosion is dominant. In the future, a better understanding of the factors controlling salt spreading around the vent and the impacts of dissolution/erosion mechanisms on the deformation will improve our ability to interpret surface deformation of the salt-caprock system at unprecedented spatial and temporal resolution.

In emergent salt diapirs, the volume of the salt at the surface represents an equilibrium state between the salt supplied from the underlying orifice and that lost by dissolution and erosion at the surface. Therefore, the salt volume at the surface represents the surplus of the salt supply over the dissolution and erosion neglecting any volume changes from the decompaction. Both dissolution and erosion processes occur at or near surface and can be estimated, while estimating salt supply remains challenging because it occurs at depth. At the surface, the salt moves away from the central dome above the diapir’s orifice toward the flanks by gravity spreading in the direction of the slope.In this work, the salt volume change at the surface was estimated using Persistent Scatterer Interferometry (PSI) data to estimate the salt supply into the surface. While the salt tends to move in the direction of slopes, maximum deformation signal can be detected from the PSI data where the slope direction aligns with the line-of-sight (LOS) of satellite in the east- and west-facing slopes of salt diapirs. The salt deformation was estimated from both the east-west and up-down components of the decomposed PSI data along an east-west trending swath profile across the symmetrical Finu salt diapir to obtain salt gain (surplus) along this profile. During the PSI processing, areas with surface erosion and dissolution (sinkholes) were mostly filtered out, and the relevant salt loss was already accounted when analyzing the salt deformation. However, the down (subsidence) signal of the analyzed PSI data in nearly horizontal areas in the flanks of the Finu diapir, where salt movement is expected to be horizontal, is assumed to be associated with the subsurface salt dissolution. The used PSI data cover a period of four years from October 2014 to December 2018.Our results show that salt area surplus is c. 14 m2 a-1 along the profile crossing the Finu diapir. With the profile length of 4.6 km, the surplus rates along it are c. 3.1 mm a-1. The average subsidence rate on the diapirs flat flanks, which is supposed to be associated with the subsurface dissolution, was estimated at c. 1.0 mm a-1. Thus, the total supply rate along the profile is c. 4.1 mm a-1. Along the profile length, these rates mean that a total section area of c. 19 m2 a-1 of salt is added to the profile. Considering the semi-circular and symmetrical shape of the diapir and assuming uniform salt supply rates, the total volume of the salt delivered to the surface is in the order of c. 70,000 m3 a-1 from the underlying orifice. The orifice is approximately circular with a diameter of c. 1.7 km and an aperture covering c. 2.3 km2 area. This means the salt is extruded at a rate of c. 30 mm a-1 averaged over the period of four years.