Continental crust formation on early Earth controlled by intrusive magmatism.

The global geodynamic regime of early Earth, which operated before the onset of plate tectonics, remains contentious. As geological and geochemical data suggest hotter Archean mantle temperature and more intense juvenile magmatism than in the present-day Earth, two crust-mantle interaction modes differing in melt eruption efficiency have been proposed: the Io-like heat-pipe tectonics regime dominated by volcanism and the "Plutonic squishy lid" tectonics regime governed by intrusive magmatism, which is thought to apply to the dynamics of Venus. Both tectonics regimes are capable of producing primordial tonalite-trondhjemite-granodiorite (TTG) continental crust but lithospheric geotherms and crust production rates as well as proportions of various TTG compositions differ greatly, which implies that the heat-pipe and Plutonic squishy lid hypotheses can be tested using natural data. Here we investigate the creation of primordial TTG-like continental crust using self-consistent numerical models of global thermochemical convection associated with magmatic processes. We show that the volcanism-dominated heat-pipe tectonics model results in cold crustal geotherms and is not able to produce Earth-like primordial continental crust. In contrast, the Plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various TTG rocks. Using a systematic parameter study, we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data (low-, medium- and high-pressure TTG). Our study thus suggests that the pre-plate-tectonics Archean Earth operated globally in the Plutonic squishy lid regime rather than in an Io-like heat-pipe regime.

Formation of ridges in a stable lithosphere in mantle convection models with a viscoplastic rheology.

Numerical simulations of mantle convection with a viscoplastic rheology usually display mobile, episodic or stagnant lid regimes. In this study, we report a new convective regime in which a ridge can form without destabilizing the surrounding lithosphere or forming subduction zones. Using simulations in 2-D spherical annulus geometry, we show that a depth-dependent yield stress is sufficient to reach this ridge only regime. This regime occurs when the friction coefficient is close to the critical value between mobile lid and stagnant lid regimes. Maps of convective regime as a function of the parameters friction coefficients and depth dependence of viscosity are provided for both basal heating and mixed heating situations. The ridge only regime appears for both pure basal heating and mixed heating mode. For basal heating, this regime can occur for all vertical viscosity contrasts, while for mixed heating, a highly viscous deep mantle is required.

Dynamic causes of the relation between area and age of the ocean floor.

The distribution of seafloor ages determines fundamental characteristics of Earth such as sea level, ocean chemistry, tectonic forces, and heat loss from the mantle. The present-day distribution suggests that subduction affects lithosphere of all ages, but this is at odds with the theory of thermal convection that predicts that subduction should happen once a critical age has been reached. We used spherical models of mantle convection to show that plate-like behavior and continents cause the seafloor area-age distribution to be representative of present-day Earth. The distribution varies in time with the creation and destruction of new plate boundaries. Our simulations suggest that the ocean floor production rate previously reached peaks that were twice the present-day value.

Comparisons between seismic Earth structures and mantle flow models based on radial correlation functions.

Three-dimensional numerical simulations were conducted of mantle convection in which flow through the transition zone is impeded by either a strong chemical change or an endothermic phase change. The temperature fields obtained from these models display a well-defined minimum in the vertical correlation length at or near the radius where the barrier is imposed, even when the fields were filtered to low angular and radial resolutions. However, evidence for such a feature is lacking in the shear-velocity models derived by seismic tomography. This comparison suggests that any stratification induced by phase or chemical changes across the mid-mantle transition zone has a relatively small effect on the large-scale circulation of mantle material.