Constraints on the recent geological evolution of Venus may be provided by quantitative estimates of the rates of the principal resurfacing processes, volcanism and tectonism. This paper focuses on the latter, using impact craters as strain indicators. The total postimpact tectonic strain lies in 0.5--6.5%, which defines a recent mean strain rate of 10-18--10-17 s-1 when divided by the mean surface age. Interpretation of the cratering record as one of pure production requires a decline in resurfacing rates at about 500 Ma (catastrophic resurfacing model). If distributed tectonic resurfacing contributed strongly before that time, as suggested by the widespread occurrence of tessera as inliers, the mean global strain rate must have been at least ~10-15 s-1, which is also typical of terrestrial active margins. Numerical calculations of the response of the lithosphere to inferred mantle convective forces were performed to test the hypothesis that a decrease in surface strain rate by at least two orders of magnitude could be caused by a steady decline in heat flow over the last billion years.

Parameterized convection models predict that the mean global thermal gradient decreases by only about 5 K/km over this time; even with the exponential dependence of viscosity upon temperature, the surface strain rate drops by little more than one order of magnitude. Strongly unsteady cooling and very low thermal gradients today are necessary to satisfy the catastrophic model. An alternative, uniformitarian resurfacing hypothesis holds that Venus is resurfaced in quasi-random ''patches'' several hundred kilometers in size that occur in response to changing mantle convection patterns. Under such a model, the observed crater strain distribution indicates that about 1% of the planet's surface is tectonically active at any time. However, this model requires a very weak crustal rheology to achieve surface velocities ~100 mm/yr appropriate to the required ''patch'' size. Without well-developed lateral weak zones, Venus is essentially a one-plate planet, but one in which the lithosphere is able to respond to topography produced by mantle convection through faulting and limiting horizontal movement. The net rate of tectonic activity is logarithmically intermediate between Earth and Mars: about 100 times slower than plate tectonics, but up to 100 times faster than planets where tectonic stress arises largely from lithospheric cooling and contraction. ¿ American Geophysical Union 1994