DRIVING FORCES FOR LIMITED TECTONICS ON VENUS

The very high correlation of geoid height and topography on Venus, alo ng with the high geoid topography ratio, can be interpreted as local i sostatic compensation and/or dynamic compensation of topography at dep ths ranging from 50 to 350 km. For local compensation within the litho sphere, the swell-push force is proportional to the first moment of th e anomalous density. Since the long-wavelength isostatic geoid height is also proportional to the first moment of the anomalous density, the swell push force is equal to the geoid height scaled by -g(2)/2 pi G. Because of this direct relationship, the style (i.e., thermal, Airy, or Pratt compensation) and depth of compensation do not need to be spe cified and can in fact vary over the surface. Phillips (1990) showed t hat this simple relationship between swell-push force and geoid also h olds for dynamic uplift by shear traction on the base of the lithosphe re caused by thermal convection of an isoviscous, infinite half-space mantle. Thus for all reasonable isostatic models and particular classe s of dynamic models, the geoid height uniquely determines the magnitud e of the swell-push body force that is applied to the venusian lithosp here. Given this body force and assuming Venus can be approximated by a uniform thickness thin elastic shell over an inviscid sphere, we cal culate the present-day global strain field using equations given in Ba nerdt (1986); areas of positive geoid height are in a state of extensi on while areas of negative geoid height are in a state of compression. The present-day model strain field is compared to global strain patte rns inferred from Magellan-derived maps of wrinkle ridges and rift zon es. Wrinkle ridges, which are believed to reflect distributed compress ive deformation, are generally confined to regions with geoid of less than 20 m while rift zones are found primarily along geoid highs. More over, much of the observed deformation matches the present-day model s train orientations suggesting that most of the rifts on Venus and many of the wrinkle ridges formed in a stress field similar to the present one. In several large regions, the present-day model strain pattern d oes not match the observations. This suggests that either the geoid ha s changed significantly since most of the strain occurred or our model assumptions are incorrect (e.g., there could be local plate boundarie s where the stress pattern is discontinuous). Since the venusian litho sphere shows evidence for limited strain, the calculation also provide s an estimate of the overall strength of the lithosphere in compressio n and extension which can be compared with rheological models of yield strength versus depth. At the crests of the major swells, where evide nce for rifting is abundant, we find that the temperature gradient mus t be at least 7 K/km. (C) 1997 Academic Press.