Abstract
What tectonic regimes operated on the early Earth and how these differed
from modern plate tectonics remain unresolved questions. We use
numerical modelling of mantle convection, melting and melt-depletion to
address how the regimes emerge under conditions spanning back from a
modern to an early Earth, when internal radiogenic heat was higher. For
Phanerozoic values of internal heat, the tectonic regime depends on the
ability of the lithosphere to yield and form plate margins. For early
Earth internal heat values, the mantle reaches higher temperatures,
high-degree depletion and differentiated into a thicker and stiffer
lithospheric mantle. This thermochemical differentiation stabilises the
lithosphere over a large range of modelled strengths, narrowing the
viable tectonic regimes of the early Earth. All the models develop in
two stages: an early stage, when decreasing yield strength favours
mobility and depletion, and a later stabilisation, when inherited
features remain preserved in the rigid lid. The thick lithosphere
reduces surface heat loss and its dependence on mantle temperature,
reconciling with the thermal history of the early Earth. When compared
to the models, the Archean record of large melting, episodic mobility
and plate margin activity, subsequently fossilised in rigid cratons, is
best explained by the two-stage evolution of a lithosphere prone to
yielding, progressively differentiating and stabilising. Thermochemical
differentiation holds the key for the evolution of Earth’s tectonics:
dehydration stiffening resisted the operation of plate margins
preserving lithospheric cores, until its waning, as radioactive heat
decays, marks the emergence of stable features of modern plate
tectonics.