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, m ost likely by crustal thickness variations. Apparent depths of isostat ic compensation, computed in the spatial domain, range from 20 to 50 k m for Alpha, Tellus, Ovda, and Thetis Regiones. Using a two-layer mode l for isostatic compensation, Alpha, Tellus, and Ovda are best represe nted as nearly completely compensated in crust that is regionally 20-4 0 km thick around these highlands, with little contribution from deepe r mantle sources. In contrast to these three areas, a stronger regiona l gravity high associated with Thetis requires a significant upper man tle component to compensation. This is evident in the spectral admitta nce as a pronounced deep, long-wavelength anomaly. In the two-layer is ostatic model, a broad, deeply compensated upland underlies a shallowl y compensated central block of Thetis. If this deep component is inter preted as a thermal anomaly, the loci of maximum upwelling agree well with sites of recent extension. Alternatively, dense mantle could be r esponsible for the broad gravity anomaly, although a different style o f dynamic coupling is called for. The plateau highlands are thus physi ographically and isostatically equivalent to terrestrial continents, t hough probably not compositionally. They also share the record of a lo ng tectonic history. The large regional gravity anomaly of Thetis indi cates that active mantle processes continue even beneath some areas (t essera) thought to be relies of a former geological regime. The excell ent 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 v enusian spreading centers. If crust was produced mostly at spreading c enters during global resurfacing, the mantle potential temperature mus t have been similar to 1800 K, about 100 K hotter than predicted. If h otspot activity dominated, lateral variations in crustal production mu st be smoothed out by creep of the lower crust. (C) 1994 Academic Pres s, Inc.