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 on Venus indicate that they are nearly random with respect to elevation. Although a slight deficit of craters at high elevations and an excess at low elevations is observed, the differences are marginally significant. Using a high-resolution digital map and database of all major volcanic, 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 crater hypsometry results from resurfacing at higher elevations by volcanic and tectonic features superimposed on less active plains. The distribution of impacts in the map units has two distinct patterns: (1) the plains and shield fields (70%) have high crater densities and low proportions of tectonized or embayed craters; and (2) the remaining volcanic and tectonic features (30%) have low crater densities and high proportions of modified craters. The plains and shield fields appear to represent a much lower level of resurfacing activity. Simple area-balance calculations indicate that resurfacing at higher elevations by tectonic and volcanic features plausibly explains the observed crater hypsometry. However, the subtlety of the effects suggests that either (1) little resurfacing has occurred during the period of crater accumulation, or (2) resurfacing acts almost equally at all elevations. The apparent low activity of the plains and their abundance at lower elevations makes it unlikely that resurfacing is balanced with respect to elevation. It appears that the plains have been mostly quiescent since their emplacement, and that subsequent resurfacing occurs mostly in the highlands as a result of volcanism, corona formation, and rifting. We estimate that since the end of plains emplacement about 14% of Venus has been resurfaced by volcanism and about 6% by tectonic deformation.