In this contribution we present a new model of passive rifting and related rift-flank uplift. The numerical model is based on a lattice spring network coupled with a viscous particle model so that we can simulate visco-elastoplastic behaviour with dynamic fault development. In our model we show that rift flank uplift can be achieved best when extension in the crust is localized and the lower crust is strong so that major rift faults transsect the whole crust. Uplift of rift flanks follows a smooth function whereas down-throw in the rift basin takes place in steps. The geometry of the developing faults has also an influence on the uplift, in this case displacement along major rift faults produces higher flanks than distributed displacement on many faults. Our model also shows that the relative elastic thickness of the crust has only a minor influence on the uplift since fault depth and elastic thickness are not independent. In addition we show with a second set of simulations and analytically that a strain misfit between the upper and lower boundaries of a stretched crust leads to an active uplift driven by elastic forces. We compare the numerical simulations, the analytical solution and real surface data from the Albertine rift in the East African Rift System and show that our new model can reproduce realistic features. Our two-layer beam model with strain misfit can also explain why a thick crust in the simulations can have an even higher rift flank than a thin crust even though the thin crust topography has a higher curvature. We discuss the implications of our simulations for real rift systems and for the current theory of rift flank uplift.
In this contribution we will explain how extreme uplift of a basement block within an extending continental rift is possible. As an example we will use the Rwenzori mountains that lie within the northern part of the western branch of the East African Rift System. This spectacular basement block was uplifted up to heights exceeding 5000m above sea level (4000m relative to its surroundings), with equatorial glaciers around the major peaks. We identified three principal mechanisms, which contribute to the Rwenzori uplift.
We used stress inversion studies based on fault slip data and numerical models to understand the stress in this region. We see that the stress field in the centre of the Rwenzori block is significantly different from the stress field of the rift flanks and the southern part of the Rwenzoris. In the central part of the mountain we find extension parallel to the main rift opening direction (NW-SE), two strike slip regimes and a NNW directed thrusting regime. In the south of the Rwenzoris we find only two orthogonal extension regimes indicating that sigma 3 and sigma 2 may have switched during the extension and activation of faults. North of the mountain extension seems to dominate with a minor strike slip component. In the NE of the mountain, where the Rwenzoris are still connected to the Tanzania craton, we find complex inclined strike slip and normal stress fields. The stress pattern suggests that the Rwenzori block and its connecting bridge contain unusual stresses probably related to block rotation and uplift. As an alternative explanation for the complex interference of strike slip. extension and even thursting one can consider the capturing of old pre-rift stress fields by the faults. However, this does not explain why the stress field varies dramatically between the rift flanks and the central mountains. In order to understand these complex stress geometries we use a three-dimensional elasto-plastic model where we implement the rift geometry and stretch it. The resulting three-dimensional stress patterns do indeed show a transition from extension to strike slip and even thrusting within the central block. We therefore argue that the stress inversion technique may give you actual recent stress-fields and that these stresses may result from the unusual uplift of the mountain and its possible rotation within the rift setting.
We present petrographic and structural analyses of a basement-hosted border fault in the East African Rift. Understanding the mechanical evolution and fluid-rock interaction of rift-flank faults is integral to developing models of fluid flow in the crust, where hydraulic connections may occur between basement faults and basin sediments. The Bwamba Fault forms the flank of the Rwenzori Mountains Horst in western Uganda, and has locally reactivated older mylonitic fabrics in the basement gneisses. The fault core features discrete mineralised and veined units. Shear fabrics and fault scarp striations indicate predominately normal kinematics, with minor strike-slip faulting and fabrics. Transient brittle failure was accompanied by two phases of fluid ingress, associated with veining and mineralisation. The first was localised and strongly influenced by host lithology. The second involved widespread Fe-oxide and jarosite mineralisation. The latter signals the onset of a hydraulic connection between Fe- and S-rich sedimentary rocks in the adjacent Semliki Rift Basin and the Bwamba Fault, involving co-seismic and or post-seismic fluid injection into the fault at ca. 150–200 °C, and 2.5–3 km depth. Such evolving permeability connections between basin sediments and basement faults are important for local hydrocarbon and geothermal systems, and may be typical of active rifts.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
