A VENUS ANALOGUE OF CATASTROPHIC MANTLE OVERTURNS ON PRECAMBRIAN EARTH

Has plate tectonics always operated in its present mode? The modern mode has operated for at least the last 1.0 to 1.5 Ga. But evidence from the early Proterozoic and Archaean is equivocal. Heat flow was 2 to 3 times the modern rate and the mantle was 100 to 200 K hotter. At potential temperatures ~60 K higher than now, the lithosphere remains positively buoyant at its subduction. At that temperature, Earth must have operated in the hindered plate regime, i.e., its plates were driven by ridge push, not slab pull. What was the tectonic environment like during that time? Cratons provide some evidence of continental and particularly collisional environments, but nothing remains of the oceans. The closest analogue to a hindered plate mode planet is Venus. Similar in size to Earth, but with a 750 K surface temperature, its lithosphere is buoyant and cannot subduct. While there are upland regions similar to terrestrial continents, most of Venus consists of low-lying plains that might be analogous Earth's Archaean and Proterozoic oceans. They are formed by continental-like rifting that generates flood lavas, diapiric intrusions, and finally localised volcanism that eventually resurfaces the thermally subsided rift. However, the hindered plate mode appears inadequate at removing heat at a sufficient rate, on either modern Venus or early Earth. Layered mantle convection might resolve this problem by allowing intervals of steady-state hindered plate tectonics that are interrupted by major convective overturns of the upper and lower mantle layers. Such overturns might explain the crater distribution on Venus and the major bursts in continental growth in the late Archaean and early Proterozoic on Earth. The effect of these events on climate and on life must have been catastrophic, but may have provided the conditions necessary for major evolutionary diversifications.