Rift induced delamination of mantle lithosphere and crustal uplift: a new mechanism for explaining Rwenzori Mountains’ extreme elevation?

Abstract

With heights of 4-5 km, the topography of Rwenzori Mountains, a large horst of old crustal rocks located inside a young passive rift system, poses the question ''Why are the Rwenzori Mountains so high?''. The Cenozoic Western Rift branch of the East African Rift System is situated within the Late Proterozoic mobile belts between the Archean Tanzania Craton and Congo Craton. 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. Our hypothesis is based on the propagation of the rift tips, surrounding the stiff old lithosphere at Rwenzori region, thereby triggering the delamination of the cold and dense mantle lithosphere (ML) root by reducing viscosity and strength of the undermost lower crust. As a result, this unloading induces fast isostatic pop-up of the less dense crustal Rwenzori block. We term this RID-''rift induced delamination of Mantle Lithosphere''. The physical consistency of the RID hypothesis is tested numerically. Viscous flow of 2D models is approximated by a Finite Difference Method with markers in an Eulerian formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component system. Based on laboratory data of appropriate rock samples, a temperature-, pressure-and stress-dependent rheology is assumed. Assuming a simple starting model with a locally heated ML, the ML block between the weakened zones becomes unstable and sinks into the asthenosphere, while the overlying continental crust rises up. Thus, RID seems to be a viable mechanism to explain geodynamically the extreme uplift. Important conditions are a thermal anomaly within the ML, a ductile lower crust with visco-plastic rheology allowing significant strength reduction and lateral density variations. The special situation of a two-sided rifting or offset rift segments to decouple the ML laterally from the surrounding continental lithosphere seems to be most decisive. Further support for the RID mechanism may come from additional crustal thickness and an extensive stress field. Some parameters, such as the excess temperature and yield stress, are very sensitive, small changes determine whether delamination takes place or not.

Crustal thinning beneath the Rwenzori region, Albertine rift, Uganda, from receiver-function analysis

“…Recent numerical simulations indicate that the mountain range was captured by two propagating rift segments leading to a separation from the rift flanks and clockwise rotation of the Rwenzori block (Koehn et al 2008). Lithospheric delamination has recently been proposed as a possible mechanism for the extreme uplift of the Rwenzori mountains (Wallner and Schmeling 2010). In this study we provide results on crustal thickness variations and velocity discontinuities beneath the Rwenzori range and the surrounding rift.…”

Thermal and exhumation history of the central Rwenzori Mountains, Western Rift of the East African Rift System, Uganda

“…As confirmed by these cooling ages, the final uplift that partly led to the formation of the recent topography must have been fast and probably started a few ka to Ma ago. Possible mechanisms allowing for this uplift are discussed by Koehn et al (2010) and Wallner and Schmeling (2010) in this issue. We assume that the erosion rate did not compensate the rock uplift rate leading to the recent high topography.…”

Rifts and Rifted Margins: A Review of Geodynamic Processes and Natural Hazards

“…During the last decade, a variety of numerical codes have been developed and 2D numerical modeling became a standard tool to investigate rift processes [e.g. Nagel and Buck, 2004;Lavier and Manatschal, 2006;Pérez-Gussinyé et al, 2006;Buiter et al, 2008;Gueydan et al, 2008;van Wijk et al, 2008;Jammes et al, 2010;Rosenbaum et al, 2010;Wallner and Schmeling, 2010;Huet et al, 2011;Huismans and Beaumont, 2011;Rey et al, 2011;Armitage et al, 2012;Beaumont and Ings, 2012;Choi and Buck, 2012;Chen et al, 2013;Chenin and Beaumont, 2013;Gueydan and Précigout, 2013;Watremez et al, 2013;Brune et al, 2014;Liao and Gerya, 2014;Clift et al, 2015;Petersen et al, 2015;Sharples et al, 2015]. The major limiting factor when conducting 3D numerical models is the model resolution.…”