AN EPISODIC HYPOTHESIS FOR VENUSIAN TECTONICS

It is suggested that episodic plate tectonics occurs on Venus; episodes of rapid plate tectonics are separated by periods of surface quiescence. For the last 500 +/- 200 m.y. it is postulated that the surface of Venus has been a single rigid plate that has been thickening due to conductive cooling. A near-uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows. A lithosphere that has conductively thickened for some 500 m.y. has a thickness of about 300 km, nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss. Such a thick lithosphere can support the high topography and associated gravity anomalies on Venus as well as the unrelaxed craters; studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere. Incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics. On the Earth, 75-90% of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges. This process is not operative on Venus. This paper suggests that episodic plate tectonics on Venus constitutes the primary mechanism for mantle heat transport on that planet.