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.ISSN:0094-8276ISSN:1944-800

Building archean cratonic roots

Geophysical, geochemical, and geological investigations have attributed the stable behaviour of Earth’s continents to the presence of their Archean cratonic roots. These roots are likely composed of melt-depleted, low density residual peridotite with high magnesium number (Mg#), while devolatilisation from the upper mantle during magmatic events might have made these roots more viscous and intrinsically stronger than the convecting mantle. Several conceptual dynamic and petrological models of craton formation have been proposed. Dynamic models invoke far-field shortening or mantle melting events, e.g., by mantle plumes, to create melt-depleted and thick cratons. Compositional buoyancy and rheological modifications have also been invoked to create long-lived stable cratonic lithosphere. However, these conceptual models have not been tested in a dynamically self-consistent model. In this study, we present global thermochemical models of craton formation with coupled core-mantle-crust evolution driven entirely by gravitational forces. Our results with melting and crustal production (both oceanic and continental) show that formation of cratonic roots can occur through naturally occurring lateral compression and thickening of the lithosphere in a self-consistent manner, without the need to invoke far-field tectonic forces. Plume impingements, and gravitational sliding creates thrusting of lithosphere to form thick, stable, and strong lithosphere that has a strong resemblance to the Archean cratons that we can still observe today at the Earth’s surface. These models also suggest the recycling of denser eclogitic crust by delamination and dripping processes. Within our computed parameter space, a variety of tectonic regimes are observed which also transition with time. Based on these results, we propose that a ridge-only regime or a sluggish-lid regime might have been active on Earth during the Archean Eon as they offer favourable dynamics and conditions for craton formation.CJ and JvH were supported by the Natural Environment Research Council under the grant NE/M000281/1. CJ is also receiving funding from the ERC Synergy Grant 856555 for the project: Monitoring Earth Evolution through Time (MEET). AR is funded by ETH Zürich.Peer reviewe

Plutonic‐Squishy Lid: A New Global Tectonic Regime Generated by Intrusive Magmatism on Earth‐Like Planets

Abstract The thermal and chemical evolution of rocky planets is controlled by their surface tectonics and magmatic processes. On Earth, magmatism is dominated by plutonism/intrusion versus volcanism/extrusion. However, the role of plutonism on planetary tectonics and long‐term evolution of rocky planets has not been systematically studied. We use numerical simulations to systematically investigate the effect of plutonism combined with eruptive volcanism. At low‐to‐intermediate intrusion efficiencies, results reproduce the three common tectonic/convective regimes as are usually obtained in simulations using a viscoplastic rheology: stagnant‐lid (a one‐plate planet), episodic (where the lithosphere is usually stagnant and sometimes overturns into the mantle), and mobile‐lid (similar to plate tectonics). At high intrusion efficiencies, we observe a new additional regime called “plutonic‐squishy lid.” This regime is characterized by a set of small, strong plates separated by warm and weak regions generated by plutonism. Eclogitic drippings and lithospheric delaminations often occur close to these weak regions, which leads to significant surface velocities toward the focus of delamination, even if subduction is not active. The location of the plate boundaries is strongly time dependent and mainly occurs in regions of magma intrusion, leading to small, ephemeral plates. The plutonic‐squishy‐lid regime is also distinctive from other regimes because it generates a thin lithosphere, which results in high conductive heat fluxes and lower internal mantle temperatures when compared to a stagnant lid. This regime has the potential to be applicable to the Early Archean Earth and present‐day Venus, as it combines elements of both protoplate tectonic and vertical tectonic models

Can Grain Size Reduction Initiate Transform Faults?—Insights From a 3-D Numerical Study

Oceanic transform faults formed at mid-ocean ridges are intrinsic features of modern plate tectonics. Nevertheless, numerical mantle convection models typically struggle to reproduce the strike-slip movement observed along transform faults on Earth. Instead, mantle convection models tend to produce mostly convergent and divergent plate boundaries. Based on regional visco-(elasto)-plastic thermomechanical models it has been demonstrated that a strong strain-induced weakening of rocks has to be assumed in order to initiate and stabilize the characteristic orthogonal ridge-transform spreading patterns. However, the physical origin of such intense rheological weakening remains unclear. Considering that in nature oceanic transform faults show a strongly reduced grain size, a potentially strong influence of grain size reduction processes on the rheological strength of these structures can be assumed. Employing 3-D thermomechanical visco-plastic models, we study the effect of grain size reduction on oceanic transform fault initiation. Our results show that ductile weakening induced by grain size reduction indeed results in sufficient localization to initiate a transform fault. Without any additional weakening mechanisms, transform faults in our models remain stable up to 2 Myr. We identify parameters that affect stability and longevity of the transform fault during the initiation phase, such as the grain damage formulation and grain growth prefactor. In our models, transform faults initiate in the brittle crust and propagate downward, thus indicating a top-down initiated localization process. The observed grain size, rheology, and strain rate inside the shear zone of our models agree well with observations in nature; however, the longevity of natural examples cannot be reached. ©2020. American Geophysical Union. All Rights Reserved

Building archean cratonic roots

Geophysical, geochemical, and geological investigations have attributed the stable behaviour of Earth’s continents to the presence of their Archean cratonic roots. These roots are likely composed of melt-depleted, low density residual peridotite with high magnesium number (Mg#), while devolatilisation from the upper mantle during magmatic events might have made these roots more viscous and intrinsically stronger than the convecting mantle. Several conceptual dynamic and petrological models of craton formation have been proposed. Dynamic models invoke far-field shortening or mantle melting events, e.g., by mantle plumes, to create melt-depleted and thick cratons. Compositional buoyancy and rheological modifications have also been invoked to create long-lived stable cratonic lithosphere. However, these conceptual models have not been tested in a dynamically self-consistent model. In this study, we present global thermochemical models of craton formation with coupled core-mantle-crust evolution driven entirely by gravitational forces. Our results with melting and crustal production (both oceanic and continental) show that formation of cratonic roots can occur through naturally occurring lateral compression and thickening of the lithosphere in a self-consistent manner, without the need to invoke far-field tectonic forces. Plume impingements, and gravitational sliding creates thrusting of lithosphere to form thick, stable, and strong lithosphere that has a strong resemblance to the Archean cratons that we can still observe today at the Earth’s surface. These models also suggest the recycling of denser eclogitic crust by delamination and dripping processes. Within our computed parameter space, a variety of tectonic regimes are observed which also transition with time. Based on these results, we propose that a ridge-only regime or a sluggish-lid regime might have been active on Earth during the Archean Eon as they offer favourable dynamics and conditions for craton formation.ISSN:2296-646

Building archean cratonic roots

Geophysical, geochemical, and geological investigations have attributed the stable behaviour of Earth’s continents to the presence of their Archean cratonic roots. These roots are likely composed of melt-depleted, low density residual peridotite with high magnesium number (Mg#), while devolatilisation from the upper mantle during magmatic events might have made these roots more viscous and intrinsically stronger than the convecting mantle. Several conceptual dynamic and petrological models of craton formation have been proposed. Dynamic models invoke far-field shortening or mantle melting events, e.g., by mantle plumes, to create melt-depleted and thick cratons. Compositional buoyancy and rheological modifications have also been invoked to create long-lived stable cratonic lithosphere. However, these conceptual models have not been tested in a dynamically self-consistent model. In this study, we present global thermochemical models of craton formation with coupled core-mantle-crust evolution driven entirely by gravitational forces. Our results with melting and crustal production (both oceanic and continental) show that formation of cratonic roots can occur through naturally occurring lateral compression and thickening of the lithosphere in a self-consistent manner, without the need to invoke far-field tectonic forces. Plume impingements, and gravitational sliding creates thrusting of lithosphere to form thick, stable, and strong lithosphere that has a strong resemblance to the Archean cratons that we can still observe today at the Earth’s surface. These models also suggest the recycling of denser eclogitic crust by delamination and dripping processes. Within our computed parameter space, a variety of tectonic regimes are observed which also transition with time. Based on these results, we propose that a ridge-only regime or a sluggish-lid regime might have been active on Earth during the Archean Eon as they offer favourable dynamics and conditions for craton formation

Mars's Crustal and Volcanic Structure Explained by Southern Giant Impact and Resulting Mantle Depletion

Mars features a crustal dichotomy, with its southern hemisphere covered by a thicker basaltic crust than its northern hemisphere. Additionally, the planet displays geologically recent volcanism only in its low latitude regions. Previous giant impact models coupled with simulations of mantle convection have shown that the crustal dichotomy can be explained by post-impact melt crystallization that emplaced a thick crust in the southern hemisphere. In this study, we show that the depleted residue left behind by the original post-impact crustal formation can spread laterally, potentially persisting beneath the northern hemisphere to the present-day. Such a large-scale mantle province would concurrently explain both the prevalence of long-term magmatism on Mars and its strong preference for localized equatorial regions.ISSN:0094-8276ISSN:1944-800

Timescales of chemical equilibrium between the convecting solid mantle and over- and underlying magma oceans

International audienceAfter accretion and formation, terrestrial planets go through at least one magma ocean episode. As the magma ocean crystallises, it creates the first layer of solid rocky mantle. Two different scenarios of magma ocean crystallisation involve that the solid mantle either (1) first appears at the core-mantle boundary and grows upwards or (2) appears at mid-mantle depth and grows in both directions. Regardless of the magma ocean freezing scenario, the composition of the solid mantle and liquid reservoirs continuously change due to fractional crystallisation. This chemical fractionation has important implications for the long-term thermo-chemical evolution of the mantle as well as its present-day dynamics and composition. In this work, we use numerical models to study convection in a solid mantle bounded at one or both boundaries by magma ocean(s) and, in particular, the related consequences for large-scale chemical fractionation. We use a parameterisation of fractional crystallisation of the magma ocean(s) and (re)melting of solid material at the interface between these reservoirs. When these crystallisation and remelting processes are taken into account, convection in the solid mantle occurs readily and is dominated by large wavelengths. Related material transfer across the mantle-magma ocean boundaries promotes chemical equilibrium and prevents extreme enrichment of the last-stage magma ocean (as would otherwise occur due to pure fractional crystallisation). The timescale of equilibration depends on the convective vigour of mantle convection and on the efficiency of material transfer between the solid mantle and magma ocean(s). For Earth, this timescale is comparable to that of magma ocean crystallisation suggested in previous studies (Lebrun et al., 2013), which may explain why the Earth's mantle is rather homogeneous in composition, as supported by geophysical constraints

Investigating the feasibility of an impact-induced Martian Dichotomy

A giant impact is commonly thought to explain the dramatic contrast in elevation and crustal thickness between the two hemispheres of Mars known as the “Martian Dichotomy”. Initially, this scenario referred to an impact in the northern hemisphere that would lead to a huge impact basin (dubbed the “Borealis Basin”), while more recent work has instead suggested a hybrid origin that produces the Dichotomy through impact-induced crust-production. The majority of these studies have relied upon impact scaling-laws inaccurate at such large-scales, however, and those that have included realistic impact models have utilised over-simplified geophysical models and neglected any material strength. Here we use a large suite of strength-including smoothed-particle hydrodynamics (SPH) impact simulations coupled with a more sophisticated geophysical scheme of crust production and primordial crust to simultaneously investigate the feasibility of a giant impact on either hemisphere of Mars to have produced its dichotomous crust distribution, and utilise spherical harmonic analysis to identify the best-fitting cases. We find that the canonical Borealis-forming impact is not possible without both excessive crust production and strong antipodal effects not seen on Mars’ southern hemisphere today. Our results instead favour an impact and subsequent localised magma ocean in the southern hemisphere that results in a thicker crust than the north upon crystallisation. Specifically, our best-fitting cases suggest that the projectile responsible for the Dichotomy-forming event was of radius 500–750 km, and collided with Mars at an impact angle of 15–30° with a velocity of 1.2–1.4 times mutual escape speed (∼6–7 km/s).ISSN:0019-103

Presentation1_Building archean cratonic roots.pdf

Geophysical, geochemical, and geological investigations have attributed the stable behaviour of Earth’s continents to the presence of their Archean cratonic roots. These roots are likely composed of melt-depleted, low density residual peridotite with high magnesium number (Mg#), while devolatilisation from the upper mantle during magmatic events might have made these roots more viscous and intrinsically stronger than the convecting mantle. Several conceptual dynamic and petrological models of craton formation have been proposed. Dynamic models invoke far-field shortening or mantle melting events, e.g., by mantle plumes, to create melt-depleted and thick cratons. Compositional buoyancy and rheological modifications have also been invoked to create long-lived stable cratonic lithosphere. However, these conceptual models have not been tested in a dynamically self-consistent model. In this study, we present global thermochemical models of craton formation with coupled core-mantle-crust evolution driven entirely by gravitational forces. Our results with melting and crustal production (both oceanic and continental) show that formation of cratonic roots can occur through naturally occurring lateral compression and thickening of the lithosphere in a self-consistent manner, without the need to invoke far-field tectonic forces. Plume impingements, and gravitational sliding creates thrusting of lithosphere to form thick, stable, and strong lithosphere that has a strong resemblance to the Archean cratons that we can still observe today at the Earth’s surface. These models also suggest the recycling of denser eclogitic crust by delamination and dripping processes. Within our computed parameter space, a variety of tectonic regimes are observed which also transition with time. Based on these results, we propose that a ridge-only regime or a sluggish-lid regime might have been active on Earth during the Archean Eon as they offer favourable dynamics and conditions for craton formation.</p