FORMATION OF FOLD-AND-THRUST BELTS ON VENUS BY THICK-SKINNED DEFORMATION

ON Venus, fold-and-thrust belts-which accommodate large-scale horizont al crustal convergence-are often located at the margins of kilometre-h igh plateaux(1-5). Such mountain belts, typically hundreds of kilometr es long and tens to hundreds of kilometres wide, surround the Lakshmi Planum plateau in the Ishtar Terra highland (Fig. 1). In explaining th e origin of fold-and-thrust belts, it is important to understand the r elative importance of thick-skinned deformation of the whole lithosphe re and thin-skinned, large-scale overthrusting of near-surface layers. Previous quantitative analyses of mountain belts on Venus have been r estricted to thin-skinned models(6-8), but this style of deformation d oes not account for the pronounced topographic highs at the plateau ed ge. We propose that the long-wavelength topography of these venusian f old-and-thrust belts is more readily explained by horizontal shortenin g of a laterally heterogeneous lithosphere. In this thick-skinned mode l, deformation within the mechanically strong outer layer of Venus con trols mountain building. Our results suggest that lateral variations i n either the thermal or mechanical structure of the interior provide a mechanism for focusing deformation due to convergent, global-scale fo rces on Venus.