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Project D2: Geodynamic modelling of extreme uplift in the Rwenzoris

Situated within a rift the 4 to 5 km topography of the Rwenzori Mountains (RWZ), poses the urgent question of what is the dynamic cause of this extreme elevation, and why was the uplift so fast within the last few million years.

The Cenozoic Western Rift branch of the East African Rift System is embedded within the Late Proterozoic mobile belts between the Archean Tanzania craton (TC) and Congo craton (CC). The special geological setting of the massif at a rift node encircled by the ends of the northern Western Rift segments of Lake Albert and Lake Edward suggests that the mechanism responsible for the high elevation of the Rwenzoris is related to the rifting process. A feedback between tectonic uplift and erosion with effects on climate patterns, ecosystems and biogeographic zone distributions up to the possible consequences for the evolution of hominids is presumed.

 

 

Fig. 1:   Schematic view of main tectonic features backed by topography. The approaching rift tips of Lake Albert - Semliki and Lake Edward - Lake George valleys nearly completely enclose the horst of Rwenzori Mountains. Volcanics, hot springs and the cluster of anomalous deep earthquakes (ADEs) are marked. A bluish arrow indicates the 2 mm/yr drift of the Victoria Plate relative to Nubian Plate.

 

 

Four hypotheses have been tested by the RiftLink team D2 to identify the geodynamic driving mechanism, for now our favorit is presented here:

 

Rift Induced Delamination (RID)

RID is proposed as a process, that is triggered by upwelling asthenosphere near the tips of two propagating and approaching rifts which have a finite offset. This situation may be met by the southward propagating Albert Rift west of the Rwenzoris and the northward propagating Edward Rift east of the Rwenzoris, see figure 1. Being encircled by the two rifts and if the lower crust under the mountain range has sufficiently low viscosity and strength, the mantle lithosphere slab beneath becomes mechanically decoupled from the neighboring stiff mantle of the Congo and the Tanzania cratons. Negative buoyancy of the cold and dense block beneath the RWZ together with weak zones associated with the propagating rifts lead to a delamination of the mantle lithosphere from the crust and detach into the asthenosphere . The less dense unloaded crustal block will then pop-up (sketched in figure 2). The cause of buoyancy forces is under debate. A anomalously low velocity zone in the lower crust beneath the southern high range may imply a less dense region entrained by partial melts.

 

 

Fig. 2:   Sketch of a vertical cut as indicated in fig. 1 with different scales for surface, crust and mantle.

 

 

Model

Viscous flow of 2D models is approximated by Finite Difference Method in an Eulerian formulation with markers. Equations of conservation of mass, momentum and energy are solved for a temperature, pressure and stress dependent rheology. For model definition see figure 3. Power law parameters of rocks representing a typical continental lithosphere are used; Byerlee's law limits strength and is reduced in the lower crust.

A Gaussian shaped strong temperature anomaly (TUA) reaching into the crust represents upwelled asthenosphere under the two rift tips and will drive the process.

 

 

Fig. 3:   Model structure, boundary conditions (left) and temperature profiles of the initial condition (right).

 

 

Results

The transient state of the model is studied after 30 kyr. Stimulus choosing this time point is a small 55 km deep cluster of earthquakes giving reason for speculations. High temperatures from the anomaly tops spread to the central axis into the lower crust below. From the initially 90 km cold central bridge in the lower crust, a narrow fraction remains connecting the detaching mantle lithosphere with the crust.

The cold lithospheric flanks adjacent to the anomalies exhibit extreme high stresses. Seismogenic regions in the mantle can be predicted and are established by observations of ADEs. Marker show upwelling asthenosphere into the anomalies and dropping cold stiff mantle lithosphere in between. Strong localized convection takes place above, ductile lower crust is entrained in the downwelling flow, hot low viscous material flows in the new opening space. Viscosity is lowest in the region between upper crust and sinking cold mantle, the cold but stressed central bridge weakens and will pinch off. This may be consistent with extremely low seismic velocities in the lower crust seen by receiver functions and observed by tomography.

The numerical method does not provide absolute vertical surface deformation because of the vertically fixed surface boundary conditions. Topography is calculated a posteriori from pressure and vertical stress at the model surface. It is adjusted to the mean of the outer 50 km, which would correspond to the level of the East African Plateau of 1.2 km. Furthermore no thickened central crust was modelled.

The high viscous upper crust behaves like an elastic cover. Because its strength is too high, it cannot break as observed in nature and thus uplift is not extreme. Nevertheless topography evolves, the central part varies around 1.2 km. Lateral valleys and outer swells can be identified as rift graben and shoulders.

 

 

Fig. 4:   Model results at time point 30 kyr, a) temperature, b) stress, c) composition and d) viscosity distributions.UC - upper crust, LC - lower crust, L - lithosphere, A - asthenosphere.

 

 

Conclusions

Delamination of lithospheric mantle under the Rwenzori crustal block seems to be a viable mechanism to explain a large part of the extreme el evation. T he associated additional uplift may be of the order of 1 - 2 km's. Important conditions therefore are a thermal anomaly, a lower crust with reducible strength and lateral density variations. For the extreme uplift the special situation of a two-sided rifting or offset rift segments to decouple the mantle lithosphere laterally from the surrounding seems to be most decisive. Some parameters, here excess temperature and yield stress, are very sensitive, small changes determine whether delamination takes place or not.

It is still an open question how to explain the remaining 2 km of topography. An promising idea seems to be a low density lower crust and a partially molten uppermost mantle, which is consistent with the low seismic velocities observed beneath the RWZ associated with volcanism and an extensive stress field.

The simple starting model is in its capability successful. In some assumptions it is too unrealistic and rough. Especially the temperature anomaly is too large and temporally too hard. Many improvements and variations concerning the temporal development of the anomaly and ascent of asthenosphere, variations of geometry, rheology, especially yield stress and more are in process.

 

 

Fig. 5:   Topography profiles at 0, 40, 50, 58, 99 kyr and 1 Myr (solid, dashed) and at 30 kyr (solid fat, Fig. 4). The hull of all profiles in 100 Myr is shown as light grey area; its border is defined by the extrema of every profile values in all time steps. Additionally the initial profile of a model without initial temperature anomaly is given (dotted).