Generation of Earth's early continents from a relatively cool Archean mantle

Several lines of evidence suggest that the Archean (4.0–2.5 Ga) mantle was hotter than today's potential temperature (TP) of 1350 °C. However, the magnitude of such difference is poorly constrained, with TP estimation spanning from 1500 to 1600 °C during the Meso‐Archean (3.2–2.8 Ga). Such differences have major implications for the interpreted mechanisms of continental crust generation on the early Earth, as their efficacy is highly sensitive to the TP. Here we integrate petrological modeling with thermomechanical simulations to understand the dynamics of crust formation during Archean. Our results predict that partial melting of primitive oceanic crust produces felsic melts with geochemical signatures matching those observed in Archean cratons from a mantle TP as low as 1450 °C thanks to lithospheric‐scale RayleighTaylor‐type instabilities. These simulations also infer the occurrence of intraplate deformation events that allow an efficient transport of crustal material into the mantle, hydrating it

Subduction metamorphism in the Himalayan ultrahigh-pressure Tso Morari massif: an integrated geodynamic and petrological modelling approach

The Tso Morari massif is one of only two regions where ultrahigh-pressure (UHP) metamorphism of subducted crust has been documented in the Himalayan Range. The tectonic evolution of the massif is enigmatic, as reported pressure estimates for peak metamorphism vary from ∼2.4 GPa to ∼4.8 GPa. This uncertainty is problematic for constructing large-scale numerical models of the early stages of India–Asia collision. To address this, we provide new constraints on the tectonothermal evolution of the massif via a combined geodynamic and petrological forward-modelling approach. A prograde-to-peak pressure–temperature–time (P–T–t) path has been derived from thermomechanical simulations tailored for Eocene subduction in the northwestern Himalaya. Phase equilibrium modelling performed along this P–T path has described the petrological evolution of felsic and mafic components of the massif crust, and shows that differences in their fluid contents would have controlled the degree of metamorphic phase transformation in each during subduction. Our model predicts that peak P–T conditions of ∼2.6–2.8 GPa and ∼600–620 ∘C, representative of 90–100 km depth (assuming lithostatic pressure), could have been reached just ∼3 Myr after the onset of subduction of continental crust. This P–T path and subduction duration correlate well with constraints reported for similar UHP eclogite in the Kaghan Valley, Pakistan Himalaya, suggesting that the northwest Himalaya contains dismembered remnants of what may have been a ∼400-km-long UHP terrane comparable in size to the Western Gneiss Region, Norway, and the Dabie–Sulu belt, China. A maximum overpressure of ∼0.5 GPa was calculated in our simulations for a homogeneous crust, although small-scale mechanical heterogeneities may produce overpressures that are larger in magnitude. Nonetheless, the extremely high pressures for peak metamorphism reported by some workers (up to 4.8 GPa) are unreliable owing to conventional thermobarometry having been performed on minerals that were likely not in equilibrium. Furthermore, diagnostic high-P mineral assemblages predicted to form in Tso Morari orthogneiss at peak metamorphism are absent from natural samples, which may reflect the widespread metastable preservation of lower-pressure assemblages in the felsic component of the crust during subduction. If common in such subducted continental terranes, this metastability calls into question the reliability of geodynamic simulations of orogenesis that are predicated on equilibrium metamorphism operating continuously throughout tectonic cycles

Subduction metamorphism in the Himalayan ultrahigh-pressure Tso Morari massif: an integrated geodynamic and petrological modelling approach

The Tso Morari massif is one of only two regions where ultrahigh-pressure (UHP) metamorphism of subducted crust has been documented in the Himalayan Range. The tectonic evolution of the massif is enigmatic, as reported pressure estimates for peak metamorphism vary from ∼2.4 GPa to ∼4.8 GPa. This uncertainty is problematic for constructing large-scale numerical models of the early stages of India–Asia collision. To address this, we provide new constraints on the tectonothermal evolution of the massif via a combined geodynamic and petrological forward-modelling approach. A prograde-to-peak pressure–temperature–time (P–T–t) path has been derived from thermomechanical simulations tailored for Eocene subduction in the northwestern Himalaya. Phase equilibrium modelling performed along this P–T path has described the petrological evolution of felsic and mafic components of the massif crust, and shows that differences in their fluid contents would have controlled the degree of metamorphic phase transformation in each during subduction. Our model predicts that peak P–T conditions of ∼2.6–2.8 GPa and ∼600–620 ∘C, representative of 90–100 km depth (assuming lithostatic pressure), could have been reached just ∼3 Myr after the onset of subduction of continental crust. This P–T path and subduction duration correlate well with constraints reported for similar UHP eclogite in the Kaghan Valley, Pakistan Himalaya, suggesting that the northwest Himalaya contains dismembered remnants of what may have been a ∼400-km-long UHP terrane comparable in size to the Western Gneiss Region, Norway, and the Dabie–Sulu belt, China. A maximum overpressure of ∼0.5 GPa was calculated in our simulations for a homogeneous crust, although small-scale mechanical heterogeneities may produce overpressures that are larger in magnitude. Nonetheless, the extremely high pressures for peak metamorphism reported by some workers (up to 4.8 GPa) are unreliable owing to conventional thermobarometry having been performed on minerals that were likely not in equilibrium. Furthermore, diagnostic high-P mineral assemblages predicted to form in Tso Morari orthogneiss at peak metamorphism are absent from natural samples, which may reflect the widespread metastable preservation of lower-pressure assemblages in the felsic component of the crust during subduction. If common in such subducted continental terranes, this metastability calls into question the reliability of geodynamic simulations of orogenesis that are predicated on equilibrium metamorphism operating continuously throughout tectonic cycles

The role of slabs and oceanic plate geometry in the net rotation of the lithosphere, trench motions, and slab return flow

Absolute plate motion models with respect to a deep mantle reference frame (e.g., hot spots) typically contain some net rotation (NR) of the lithosphere. Global mantle flow models for the present-day plate setting reproduce similarly oriented NRs but with amplitudes significantly smaller than those found in some high NR Pacific hot spot reference frames. It is therefore important to understand the mechanisms of NR excitation, which we attempt here with two-dimensional cylindrical models of an idealized Pacific domain. We study the influence of slab properties, oceanic ridge position, continental keels, and a weak asthenospheric layer on NR and trench migration. Fast slab return flow develops in models with stiff slabs and moderate slab dips. Rapid NRs, comparable to the high NR Pacific hot spot reference frames, are primarily induced by asymmetric slab dips, in particular a shallow slab beneath South America and a steep slab in the western Pacific. A scaling relationship links the amplitude of NR to plate size, slab dip angle, and slab viscosity. Asymmetric ridge positions also promote NR through asymmetric plate sizes. Continental keels have less impact, in contrast to what has been found in earlier global studies. Several models yield unidirectional Pacific trench motions, such as slab advance in the western Pacific and, simultaneously, slab retreat in the eastern Pacific. Our model provides a physical explanation for NR generation in the present-day Pacific setting and hints at mechanisms for the temporal evolution of the basin. RI Becker, Thorsten/A-6665-2010; Moresi, Louis/H-1390-201