CONSTRAINTS ON THE RESURFACING HISTORY OF VENUS FROM THE HYPSOMETRY AND DISTRIBUTION OF VOLCANISM, TECTONISM, AND IMPACT CRATERS

Improved measurements of the target elevations of 885 impact craters o n Venus indicate that they are nearly random with respect to elevation . Although a slight deficit of craters at high elevations and an exces s at low elevations is observed, the differences are marginally signif icant. Using a high-resolution digital map and database of all major v olcanic, tectonic and impact features, we examine the distribution of impacts within volcanic and tectonic features, and the distribution of volcanism and tectonism with elevation. We show that the observed cra ter hypsometry results from resurfacing at higher elevations by volcan ic and tectonic features superimposed on less active plains. The distr ibution of impacts in the map units has two distinct patterns: (1) the plains and shield fields (70%) have high crater densities and low pro portions of tectonized or embayed craters; and (2) the remaining volca nic and tectonic features (30%) have low crater densities and high pro portions of modified craters. The plains and shield fields appear to r epresent a much lower level of resurfacing activity. Simple area-balan ce calculations indicate that resurfacing at higher elevations by tect onic and volcanic features plausibly explains the observed crater hyps ometry. However, the subtlety of the effects suggests that either(1) l ittle resurfacing has occurred during the period of crater accumulatio n, or (2) resurfacing acts almost equally at all elevations. The appar ent low activity of the plains and their abundance at lower elevations makes it unlikely that resurfacing is balanced with respect to elevat ion, It appears that the plains have been mostly quiescent since their emplacement, and that subsequent resurfacing occurs mostly in the hig hlands as a result of volcanism, corona formation, and rifting. We est imate that since the end of plains emplacement about 14% of Venus has been resurfaced by volcanism and about 6% by tectonic deformation.