Floor subsidence and rebound of large Venus craters

The topography and geology of large craters on Venus reveal no evidence for floor rebound or relaxation; rather, the floors have subsided. Depressions of the floor centers relative to the margins are evident in topography, and floor faulting is interpreted as contractional failure. Of the likely processes responsible, only subsolidus thermal contraction applies to craters that both have and have not been infilled by lavas, assuming volcanism occurred within a few tens of millions of years 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 10(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 only on transient crater diameter, but also on gravity and target density. The unfractured melt sheets of three large, young, bright-floored craters imply sufficient lithospheric rigidity to support the crater cavities. 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 for early rebound in older, dark-floored craters may have been buried by subsequent volcanism.