FLOOR SUBSIDENCE AND REBOUND OF LARGE VENUS CRATERS

The topography and geology of large craters on Venus reveal no evidenc e for floor rebound or relaxation; rather, the floors have subsided. D epressions of the floor centers relative to the margins are evident in topography, and floor faulting is interpreted as contractional failur e. Of the likely processes responsible, only subsolidus thermal contra ction applies to craters that both have and have not been infilled by lavas, assuming volcanism occurred within a few tens of millions of ye ars after the impact. Thermal subsidence satisfies the measured floor depressions for reasonable scaling of impact energies and temperature distributions in the lithosphere. Further, the predicted stresses are generally consistent with observed floor fracturing. We constrain the impact heat deposited in the lithosphere to be less than roughly 5 x 1 0(23) J for diameters of similar to 100 km. The absence of perceptible floor subsidence at craters this size on the Moon and icy satellites is readily explained by the scaling dependence of impact energy not on ly on transient crater diameter, but also on gravity and target densit y. The unfractured melt sheets of three large, young, bright-floored c raters imply sufficient lithospheric rigidity to support the crater ca vities. An elastic flexural rebound model restricts the elastic plate thickness to at least 10-15 km for the three craters, corresponding to maximum geotherms of similar to 20-30 K km(-1). Structural evidence f or early rebound in older, dark-floored craters may have been buried b y subsequent volcanism.