WHEN DID PLATE TECTONICS BEGIN? TEN YEARS AFTER THE LANDER PENROSE CONFERENCE

It has been nearly10 years since the Lander WY Penrose Conference in June 2006, and we still have many possibilities as to when plate tectonics (PT) began on Earth. Most recent progress on this question has come from geodynamic modeling and Hf and Nd isotopes. Geologic, geochemical and isotopic evidence suggests PT began by no later than 3 Ga and was going strong by 2 Ga, whereas geodynamic modeling allows starting times between 2.5 and 4 Ga or even earlier. Yet the first occurrences of ophiolites, blueschists, and UHP metamorphism do not appear until after 1 Ga. Combining results from geologic, geochemical and model studies it would appear that PT did not begin with bang but evolved from a stagnant lid regime beginning around 3 Ga (or earlier) with an episodic transitional period lasting until about 2 Ga characterized by bursts of PT activity, the most widespread of which occurred at 2.7 Ga. The last stagnant lid may have been during the crustal age gap at 2.4-2.2 Ga. As plates strengthened due to cooling of the mantle, Precambrian-style PT evolved into modern-style PT between 1 and 0.5 Ga. This evolving terrestrial history is important in searching for exoplanets with PT and life, in that it would appear that PT is not an all or nothing regime, but that rocky planets evolve between stagnant lid and PT regimes perhaps more than once during their lifetimes.