Tessera deformation and the contemporaneous thermal state of the plateau highlands, Venus

Abstract

Geodynamic modeling of tessera deformation at the ancient Venusian plateau highlands implies lithospheric thermal gradients exceeding 17 K km −1. Regularly spaced contractional ridges in the highlands manifest the unstable growth of perturbations at a dominant wavelength of ≤ 20 km controlled by a mechanical layer of relatively uniform thickness. We model the deformation as the shortening of a plastic layer over a viscous substrate; a layer thickness of less than 6 km is consistent with the observed ridge spacings. Assuming an anhydrous diabase crustal rheology and a minimum strain rate of 10 −16 S −1, the geotherm was at least 17 K km −1. Higher strain rates — consequently higher thermal gradients — are more probable. We rule out the possibility that the plateau highlands were locally anomalously hot at the time ridges formed in the tessera, that the highlands are dominantly composed of rock more felsic than diabase, or that the crust was weak because it was hydrated. Therefore, the average global lithospheric heat flux when the highlands tesserae were deforming was considerably higher than it has been since.