WHEN DID PLATE TECTONICS INITIATE?

The plate tectonic force balance is broadly understood and subduction initiation appears to be a robust manifestation of the modern system. However, when and how it began are not. The principal hurdle to attaining mobile lid tectonics is overcoming the strength of the lithosphere, but this raises a chicken-or-egg paradox: what caused this strong layer to generate weakened planar features on which subduction could initiate? Answering this question will require knowledge of the interrelationships among the full range of relevant lithospheric rheologies but at present few firm conclusions can be drawn. What is clear is that even such models will never achieve ab initio reconstruction of early Earth. Mantle convection, a highly non-linear, dispersive, chaotic system, is uninvertible and the recent recognition that global tectonic mode is pathway dependent only underscores this limitation. If we’re ever to understand when plate tectonics got underway, we’re going to have to acquire that knowledge from the geologic record. Speculations on when plate tectonics initiated have been made since the dawn of the revolution. All estimates rest on assumptions that can be grouped into five types: 1) preservation of modern plate tectonic features, 2) detrital zircon age spectra, 3) trace element and isotope geochemistry, 4) atmosphere-crust-mantle exchange, and 5) model-based estimates. While each approach has specific limitations, all share the flaw that, even if demonstrable evidence is obtained for plate tectonics activity at a specified time, the fragmentary geologic record potentially masks earlier episodes that had previously terminated. The late, great geologist Kevin Burke argued that the null hypothesis coupled with the broad acceptance of modern plate tectonics puts the onus on demonstrating when plate tectonics was not extant on Earth rather than a guilty-until-proven-innocent approach. From that perspective, the evidence at hand supports the null hypothesis back to nearly 4.5 Ga. However, an alternate null hypothesis arising from comparative planetology might be that stagnant lid convection is the normal tectonic mode on active planetary bodies with a silicate mantle, and that any such object should be assumed to be in stagnant lid mode unless strong evidence for mobile lid convection exists.