RECENT DEFORMATION RATES ON VENUS

Constraints on the recent geological evolution of Venus may be provide d 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 tecton ic strain lies in the range 0.5-6.5%, which defines a recent mean stra in rate of 10(-18)-10(-17) s(-1) when divided by the mean surface age. Interpretation of the cratering record as s one of pure production re quires a decline in resurfacing rates at about 500 Ma (catastrophic re surfacing model). If distributed tectonic resurfacing contributed stro ngly before that time, as suggested by the widespread occurrence of te ssera as inliers. the mean global strain rate must have been at least similar to 10(-5) s(-1), which is also typical of terrestrial active m argins. Numerical calculations of the response of the lithosphere to i nferred mantle convective forces were performed to test the hypothesis that a decrease in surface strain rate by at least two orders of magn itude could be caused by a steady decline in heat flow over the last b illion years. Parameterized convection models predict that the mean gl obal thermal gradient decreases by only about 5 K/km over this time; e ven 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 ne cessary to satisfy the catastrophic model. An alternative, uniformitar ian resurfacing hypothesis holds that Venus is resurfaced in quasi-ran dom ''patches'' several hundred kilometers in size that occur in respo nse to changing mantle convection patterns. Under such a model, the ob served crater strain distribution indicates that about 1% of the plane t's surface is tectonically active at any time. However, this model re quires a very weak crustal theology to achieve surface velocities simi lar to 100 mm/yr appropriate to the required ''patch'' size. Without w ell-developed lateral weak zones, Venus is essentially a one-plate pla net, but one in which the lithosphere is able to respond to topography produced by mantle convection through faulting and limited horizontal movement. The net rate of tectonic activity is logarithmically interm ediate between Earth and Mars: about 100 times slower than plate tecto nics, but up to 100 times faster than planets where tectonic stress ar ises largely from lithospheric cooling and contraction.