EARTH
In the literature: contested
When did modern plate tectonics begin on Earth?
In plain words
Proposed start dates range from more than 4 billion years ago to under 1 billion years ago. Old rocks are scarce and can be read in more than one way.
Precise statement
Date the onset of a global mobile-lid regime with deep subduction. Proposals range from about 4.2 Ga (Hadean zircon chemistry) through about 3.2 to 2.5 Ga (geochemical transitions, paleomagnetic relative motion) to about 0.8 Ga (first widespread blueschists and ultrahigh-pressure rocks). An answer is an onset age, or a demonstration of continuous operation since the Hadean, consistent with petrological, geochemical and paleomagnetic records.
What would settle it
Paleomagnetic proof of differential motion between Archean cratons together with petrological evidence of cold, deep subduction in dated rocks.