6. Plate Boundary Interactions Through Geologic History

Estimates of when plate tectonics began range from the last 20% of Earth history to within the first 5%. While there is no observation that precludes plate tectonics from operating at 4.3 Ga, evidence that it was is indirect. Although subduction initiation is a robust feature of the modern plate tectonic system and we can calculate with some accuracy when oceanic lithosphere attains negative buoyancy, we don’t yet understand how strong the lithosphere weakens sufficiently for subduction to initiate. Most approaches used to estimate when Earth first entered the mobile lid regime—preservation of modern plate tectonic features, detrital zircon age spectra, trace element and radiogenic isotope geochemistry, atmosphere-crust-mantle exchange, and model-based estimates—can be interpreted in multiple ways and are all underlain by assumptions that cannot be independently tested. All share the flaw that absence of evidence is not evidence of absence. Of special concern is that the Precambrian geologic record is likely biased to rock compositions most likely to resist deformation and thus exposure to erosion at newly rifted continental margins where loss to subduction erosion could occur. Thus any look-back comparison is flawed to some degree by a preservation bias. A more recently recognized limitation is the failure to consider how a hotter, early Earth would differ petrologically from, say, Phanerozoic behavior (e.g., lower incompatible trace element concentrations in mantle magmas, higher geothermal gradients). Historically, computational limitations in early geophysical modelling methods led to skepticism regarding the possibility of plate tectonics on early Earth. Influenced by this view, the geologic community was reluctant to take a dynamic view of the preserved crustal record, instead inferring the apparent absence of a Hadean rock record as evidence that there never was one. The unknown extent to which ancient continental crust was recycled into the mantle and thoroughly mixed, the abovementioned selection biases in the rock record, and the assumption of uniformitarian conditions throughout Earth history limit virtually all continental growth estimates to providing only lower age bounds and thus minimum estimate on the initiation age of subduction.