The Cenozoic evolution of the Himalaya-Tibet Plateau, dictated by the India-Asia convergence, remains a subject of substantial ambiguity. Here, a thermo-mechanical model is used to show the critical controls of decelerating convergence on the formation and stabilization of distinctive tectonic structures during prolonged collision. At high constant convergence rates, similar to the late Paleogene India-Asia motions, the lower plate crust is injected beneath the overriding crust, uplifting a plateau, first, then is exhumed towards the orogeny front. Conversely, low constant convergence rates, similar to the Neogene India-Asia motions, induce crustal thickening and plateau formation without underplating or exhumation of incoming crust. Strikingly, models simulating the decelerating India-Asia convergence history portray a dynamic evolution, highlighting the transitory nature of features under decreasing convergence, as the orogen shifts to a new equilibrium. In the transitional phase, the slowing of convergence decreases basal shearing and compression, leading to extension and heating in the orogen interiors. This allows diapiric ascent of buried crust and plateau collapse, as accretion migrates to a frontal fold-and-thrust belt. The models provide insights into the multi-stage evolution of the long-lived Himalayan-Tibetan orogeny, from fast early growth of the Tibetan Plateau, through its transient destabilisation and late-stage internal extension, behind the expanding Himalayan belt.

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.

Cratons are stable parts of the Earth’s continental lithosphere that have remained largely undeformed for several billion years. These consist of crustal granite-greenstone terrains coupled to roots of strong, buoyant cratonic lithospheric mantle that extend up to several hundreds of kms depth. Due to their stability, cratons preserve a record of the tectonics and the thermal evolution of the mantle in the early Earth. These observations suggest that the highly viscous (strong) character of cratons hampered the viability of early Earth tectonics, thereby affecting mantle convection patterns and cooling. In this study, we investigate the controls of stiff cratons on the initiation of subduction and mantle thermal evolution on the early Earth. Using numerical models, we simulate the effects of strong and buoyant cratons on mantle convection. We vary a set of parameters including (i) width and thickness of cratons, and (ii) viscosity ratio between cratonic lithosphere and cratonic crust. We test initial conditions varying the number of cratons, which is unconstrained for early Earth and associated it to mantle cooling rates. Our preliminary results show that the mantle cooling rate decreases with increasing number of cratons. Because mantle cooling rates affect the early Earth transition from a basaltic drip regime to initiation of subduction, we show that the craton coverage on the early Earth controls the time of onset of plate tectonics. Furthermore, we observe that cratons will remain separate or combine depending on the convective cell size, which is function of mantle cooling.