Pulsating continents on Venus: An explanation for crustal plateaus and tessera terrains

Abstract

We propose that tessera terrains on Venus represent continental crust that does not participate in the periodic recycling of the lithosphere through global subduction events.We have studied the force balance on the boundary of a continental area that survives a global subduction event using an analytical model. In the proposed model, the ratio between the crustal and lithospheric mantle thicknesses controls the force balance. If the crust thickness is less than ∼2/5 of the lithospheric mantle thickness, the continental area will be compressed, but if the crustal thickness is higher than ∼2/5 of the lithospheric mantle thickness, the continental area will spread out and collapse. Consequently, if the lithospheric mantle beneath a continental region is delaminated during a global subduction event, the continent will collapse generating tessera inliers dominated by extensional tectonics. But if a significant portion of lithospheric mantle remains, then the continental area will be compressed generating a plateau by crustal shortening. The observed plateau heights can be explained by this model, a ≈2 km height plateau can be generated by a lithospheric mantle thickness of 40 km while a ≈4 km height plateau can be generated by a 90 km thick lithospheric mantle. We have modelled this crustal thickening of a continental area by tectonic contraction using a thin viscous sheet approach with a Newtonian viscosity for the crust. The force from a hot mantle elevated during a global subduction event is enough to build up a plateau by compression in ∼50 Ma using a viscosity for the continental crust of η=1021 Pa s and ∼200 Ma for η=5·1021 Pa s. During this compressional stage concentric fold and thrust belts are generated in the plateau-continent, erasing any impact craters that were present. The subsequent stabilization of a new crust and lithosphere in the surrounding mantle changes the force balance allowing a moderate gravitational collapse of the plateau-continent accommodated by radial grabens. The pulsating continent model links for the first time the generation of crustal plateaus and the origin of the volcanic plains predicting the observed equivalent effective crater density for both terrains.