TESSERA DEFORMATION AND THE CONTEMPORANEOUS THERMAL STATE OF THE PLATEAU HIGHLANDS, VENUS

Geodynamic modeling of tessera deformation at the ancient Venusian pla teau highlands implies lithospheric thermal gradients exceeding 17 K k m(-1). Regularly spaced contractional ridges in the highlands manifest the unstable growth of perturbations at a dominant wavelength of less than or equal to 20 km controlled by a mechanical layer of relatively uniform thickness. We model the deformation as the shortening of a pl astic layer over a viscous substrate; a layer thickness of less than 6 km is consistent with the observed ridge spacings. Assuming an anhydr ous 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 -- conse quently higher thermal gradients -- are more probable. We rule out the possibility that the plateau highlands were locally anomalously hot a t the time ridges formed in the tessera, that the highlands are domina ntly composed of rock more felsic than diabase, or that the crust was weak because it was hydrated. Therefore, the average global lithospher ic heat flux when the highlands tesserae were deforming was considerab ly higher than it has been since.