HIGHLAND TESSERAE ON VENUS AS TECTONIC EQUIVALENTS TO CRATONS ON EARTH

Highland tesserae on Venus share several important traits with cratons on Earth. Both terrain types are associated with thicker than average crust, record extensive tectonic deformation, and are typically the oldest material where they are preserved. Most of Earth’s cratons reached their modern, stable form from the late Proterozoic to the early Cambrian, although they feature components that were originally cratons in their own right that date from the Archean and early Proterozoic. Whereas cratonic rocks on Earth are dominantly felsic, there is no direct evidence for evolved rocks on Venus. Yet tesserae bear similarities to continental crust on the basis of geophysical properties and infrared emissivity. On Earth, the mantle lithospheric roots of cratons have been depleted by partial melt extraction and so have lower density and markedly higher viscosity than oceanic mantle, along with lower abundances of heat-producing elements and lower heat flux at the base of the crust. On Venus, long-wavelength gravity and topography reveal mass deficits in the mantle beneath highland tesserae, but whether those deficits result from thermal or compositional differences is not known. Earth’s cratons have been shaped by plate tectonic processes and reflect the collision and accretion of distinct tectonic units. For Venus, there is structural evidence that several highland tesserae were assembled from smaller pieces of originally separated tesserae, implying large-scale lateral mobility of the crust. And elsewhere on Venus, stratigraphically young plains have been tectonically deformed so as to acquire the morphological characteristics of adjacent tesserae—implying that, in places, tessera area and volume expanded with associated uplift and crustal thickening, processes also documented at the margins of cratons. It is not clear if Venus once operated under a mobile-lid regime, but climate evolution models for the planet are permissive of an earlier era in which surface water was stable, a condition thought to be key to plate recycling on Earth. If highland tesserae are the tectonic equivalents of Earth’s cratons, they represent a second record of early geodynamics on an Earth-like world—offering not only insight into Venus’ geological history, but a new basis for understanding Archaean Earth, as well.