Numerical simulation of subcontinent
lithosphere dynamics: craton stability,
evolution and formation

Abstract

Through geodynamical modelling, two hypotheses about the craton stability and evolution were
revisited and an important process of cratonization is investigated. Unlike most previous, related
numerical studies, non-Newtonian rheology with composition dependence was used in these
studies, and the rheological parameters are thus directly comparable with laboratory experiment
of mantle. The first hypothesis, that the cratonic lithosphere is “isopycnic”, is found to be not
strictly necessary for craton stability and longevity. The high viscosity of the cratonic litho-
sphere due to compositional effects on the mantle rheology is found to be essential to maintain a
thickness difference between cratonic and non-cratonic lithosphere for over billions of years and
it allows a modest negative buoyancy of the cratonic root, depending on the strengthening factor
due to the compositional effects. The second hypothesis to be tested is that mantle plume im-
pingements cause rapid, significant removal of subcontinental lithosphere. The results presented
in this thesis show that the erosion caused by a plume impact on a continent that is strong
enough to have survived billions of years of Earth’s history is rather limited. A special weaken-
ing mechanism of such highly viscous and buoyant roots is required to reactivate this cratonic
lithosphere and thus cause significant thinning within 10s of Myrs. The fluid/melt-rock interac-
tion during mantle metasomatism is probably the most likely mechanism to modify and weaken
depleted cratonic lithosphere. Therefore, metasomatic weakening is essential for the significant
thinning of subcontinental lithosphere observed, e.g.at North China Craton and Namibia, south-
ern African, no matter whether caused by a plume impact or another tectonic event.
Using the reasonable compositional effects on the buoyancy and rheology of mantle rocks
from the above studies, numerical experiments are performed to study the formation of thick
cratonic lithosphere from a layered, depleted mantle material. In this scenario, substantial tec-
tonic shortening and thickening of previously depleted material seems to be an essential ingre-
dient to initiate the cratonization process. Afterwards, gravitational self-thickening will cause
further thickening. Compositional buoyancy resists Rayleigh-Taylor instability collapse and
stabilizes the thick cratonic root, while the secular cooling also has a stabilizing effect on the
cratonic root by reducing the thermal buoyancy contrast between lithosphere and asthenosphere
and increasing mantle viscosity. The presented numerical results are consistent with the vertical
movement of cratonic peridotite as suggested on petrological grounds.