THE DEEP-STRUCTURE OF VENUSIAN PLATEAU HIGHLANDS

Magellan gravity data confirm that several of the large, tectonically deformed, plateau-like highlands on Venus are shallowly compensated, most likely by crustal thickness variations. Apparent depths of isostatic compensation, computed in the spatial domain, range from 20 to 50 km for Alpha, Tellus, Ovda, and Thetis Regiones. Using a two-layer model for isostatic compensation, Alpha, Tellus, and Ovda are best represented as nearly completely compensated in crust that is regionally 20-40 km thick around these highlands, with little contribution from deeper mantle sources. In contrast to these three areas, a stronger regional gravity high associated with Thetis requires a significant upper mantle component to compensation. This is evident in the spectral admittance as a pronounced deep, long-wavelength anomaly. In the two-layer isostatic model, a broad, deeply compensated upland underlies a shallowly compensated central block of Thetis. If this deep component is interpreted as a thermal anomaly, the loci of maximum upwelling agree well with sites of recent extension. Alternatively, dense mantle could be responsible for the broad gravity anomaly, although a different style of dynamic coupling is called for. The plateau highlands are thus physiographically and isostatically equivalent to terrestrial continents, though probably not compositionally. They also share the record of a long tectonic history. The large regional gravity anomaly of Thetis indicates that active mantle processes continue even beneath some areas (tessera) thought to be relies of a former geological regime. The excellent agreement of modeled crustal thicknesses around Alpha, Tellus, and Ovda Regiones suggests that 20-40 km is a representative global value for the plains. Such a crust is thicker than previously estimated and about twice as thick as the expected thickness of crust produced at venusian spreading centers. If crust was produced mostly at spreading centers during global resurfacing, the mantle potential temperature must have been similar to 1800 K, about 100 K hotter than predicted. If hotspot activity dominated, lateral variations in crustal production must be smoothed out by creep of the lower crust. (C) 1994 Academic Press, Inc.