VENUS - VERTICAL ACCRETION OF CRUST AND DEPLETED MANTLE AND IMPLICATIONS FOR GEOLOGICAL HISTORY AND PROCESSES

Models for the vertical accretion of a basaltic crust and depleted man tle layer on Venus over geologic time predict the eventual development of a net negatively buoyant depleted mantle layer, its foundering and its remixing with the underlying mantle. The consequences of the deve lopment of this layer, its loss, and the aftermath are investigated an d compared to the geologic record of Venus revealed by Magellan. The y oung average age of the surface of Venus (several hundred million year s), the formation of the heavily deformed tessera regions, the subsequ ent emplacement of widespread volcanic plains, the presently low rate of volcanic activity, an impact crater population that cannot be disti nguished from a completely spatially random distribution, and the smal l number of impact craters embayed by volcanism, are all consistent wi th the development of a depleted mantle layer, its relatively rapid lo ss followed by large-scale volcanic flooding, and its subsequent reest ablishment. We outline a 'catastrophic' tectonic resurfacing model in which the foundering of the depleted mantle layer several hundred mill ion years ago caused globally extensive tectonic deformation and oblit eration of the cratering record, accompanied by upwelling of warm fert ile mantle and its pressure-release melting to produce extensive surfa ce volcanism in the following period. Venus presently appears to be ch aracterized by a relatively thick depleted mantle layer and lithospher e reestablished over the last several hundred million years following the previous instability event inferred to have produced the tessera t errain.