Journal of Conference Abstracts

FEM Modelling of Convective Upwelling of Mantle Lithosphere beneath Rifts and Other Zones of Thermally Young Lithosphere; Mantle Lithosphere Rayleigh-Taylor (R-T) Instability

Ritske Huismans (huir@geo.vu.nl)¹, Yuri Podladchikov (yura@erdw.ethz.ch)² & Sierd Cloetingh (Cloetingh@geo.vu.nl)¹

¹Inst. Earth Sciences, Vrije Universiteit, de Boelelaan 1085, Amsterdam, 1081 HV, the Netherlands

²Geologisches Institut, ETH-Zurich, Sonneggstr. 5, Zurich, CH 8092, Switzerland

We present quantitative modelling results of the dynamic interplay of passive extension and active convective (R-T) thinning of the mantle lithosphere beneath intra-continental rift zones. We investigate the relative importance of thermal buoyancy forces associated with asthenospheric doming and far-field intra-plate stresses on the style of rifting. To this aim we employ a two-dimensional numerical code based on a FEM formulation for non-linear temperature dependent visco-elasto-plastic rheology. The models support a scenario in which passive stretching leads to an unstable lithospheric configuration. Thermal buoyancy related to this asthenospheric doming subsequently drives active upwelling in a lithospheric scale convection cell. In the late syn-rift to early post-rift the lithospheric horizontal stresses caused by the active asthenospheric upwelling start to compete with the far-field intra-plate stresses. At this stage the domal forces may dominate and even drive the system causing a change from passive to active rifting mode. If this transition occurs, the numerical models predict 1) drastic increase of sub-crustal thinning beneath the rift zone, 2) lower crustal flow towards the rift flank, 3) middle crustal flow towards the rift centre, 4) the coeval occurrence of tensional stresses within and compressive stresses around the upwelling region, 5) possible surface uplift. Late post rift thermal cooling removes the thermal buoyancy forces. At this stage, the far-field forces dominate the stress state again and the lithosphere becomes more sensitive to small changes in the intra-plate stresses. The model results may explain several key observations that are characteristic of a large number of intra-continental rift basins. These features include differential thinning of extending lithosphere, the discrepancy between fault related extension and crustal thinning, late (end of syn-rift ­ early post-rift) mantle related volcanism, surface "domal" uplift succeeding rifting, rift flanks uplift associated with extension of a weak lithosphere (shallow level of necking case), and late stage acceleration of subsidence caused by compressive intraplate stresses.

Large scale thermal disturbances of the shallow asthenosphere have been invoked to explain the observation of coeval acceleration of rifting of basins over large areas and the isotopic and geochemical similarity of the rift related alkaline volcanics. For example the Late Devonian rifting of the East European Platform (Dnepr-Donets ea.) and the Miocene age West and Central European rift system. However, the distributed nature of uplifting regions, rifting, and alkaline volcanism suggest that the thermal structure and the stress state of the lithosphere play a major role in determining where and how a thermal anomalous shallow asthenosphere influences the activation of rifts and the generation of alkaline volcanism. We propose that an increase of the asthenosphere potential temperature may facilitate R-T instable growth of variations in the lithosphere thickness and may induce acceleration of already active but not very efficient rifts, in this way producing a transition from passive to active rifting.