| Impact cratering has been recognized as an important process re-shaping planetary surfaces. Here, I use impact crater morphology to evaluate the geological processes that have taken place on Venus and Mars. Various features on Venus have been attributed to upwellings: regiones, broad topographic rises extending 1,000's of kilometers; radial graben-fissure systems extending 100's of km; and smaller, quasi-circular uplifted "coronae". For each of these structures, alternative explanations have been proposed. Venus hosts nearly 1,000 impact craters, which indicates that the planet was resurfaced ∼400--800 m.y. ago. A minority of craters have been modified by tectonic and/or volcanic activity. Using the impact crater distribution and modification, I assess competing explanations for each of the suggested plume-related features in the Beta-Atla-Themis (BAT) region.; The BAT region includes the planetary geoid and topographic highs, profuse volcanism, the intersection of three major rifts, and numerous coronae. Although just one-sixth of the surface, it contains 61% of the craters that are both tectonized and embayed. Using Magellan radar and altimetry data to establish uplift and deformation, detailed interpretations are given for several craters. Crater orientation indicates local tilting. Most impact craters dip away from Atla's geoid high, but on Beta dip directions are more random. Within radial systems, minimal volcanic modification of craters has occurred, and crater dips do not suggest recent uplift. Modification and deficit of craters near and within coronae indicate that volcano-tectonic processes, possibly plumes, form coronae and may still be active.; The presence of radar-dark parabolic deposits associated with ∼60 craters, including Von Schuurman on Atla Regio, dates these as the youngest impact features on Venus. Significantly, a hint of Von Schuurman's parabola is apparent in the stereo-derived topography. A measure of the thickness of this deposit could constrain models for impact interaction with the atmosphere.; Finally, I examine the nature of bulges found within three similar sized, mid-latitude craters on Mars. Changes in crater morphology are thought to reflect the impact energy and target properties involved in the crater formation. The origin of these bulges may clarify the interaction of crustal material and the impact process on Mars' younger plains. |
