Periodic Reporting for period 2 - CRUSLID (Formation, magmatic evolution and present-day structure of the CRUsts of Stagnant-LID planets)
Berichtszeitraum: 2023-04-01 bis 2024-09-30
The Earth is the only planet in our solar system showing plate tectonics with two distinct types of plates: ancient, buoyant continents and continuously forming and recycling, young, oceanic plates.
Studies have revealed that the low density of continents is caused by processes that affected the distribution of material throughout the continents.
Whether plate tectonics – the movement of Earth's lithosphere –have been driven originally by the low density of the continents or whether this low density was a result of plate tectonics processes is one of the major scientific issues in Earth Sciences.
With the growing number of space missions having studied the other planets of our solar system, we have acquired a large number of data and observations on the surface and internal structure of the terrestrial planets of our solar system, which do not show plate tectonics.
However, gaps remain in our knowledge regarding the characteristics of the crust and lithosphere on these one-plate planets and how they formed. In particular, do long-wavelength lateral variations in the structure of the plate of one-plate planets exist? Could similar physical processes have affected the distribution of material in the crust of planets with no plate tectonics?
Coupling fluid dynamical models of crust formation and magmatism as well as planetary thermal evolution models with geological and geophysical observations on the internal structure and surface composition of one-plate planets, the EU-funded CRUSLID project seeks to understand the formation and subsequent evolution of planetary crusts and lithospheres on the Moon, Mars and Venus from a physical perspective.
Studies have revealed that the low density of continents is caused by processes that affected the distribution of material throughout the continents.
Whether plate tectonics – the movement of Earth's lithosphere –have been driven originally by the low density of the continents or whether this low density was a result of plate tectonics processes is one of the major scientific issues in Earth Sciences.
With the growing number of space missions having studied the other planets of our solar system, we have acquired a large number of data and observations on the surface and internal structure of the terrestrial planets of our solar system, which do not show plate tectonics.
However, gaps remain in our knowledge regarding the characteristics of the crust and lithosphere on these one-plate planets and how they formed. In particular, do long-wavelength lateral variations in the structure of the plate of one-plate planets exist? Could similar physical processes have affected the distribution of material in the crust of planets with no plate tectonics?
Coupling fluid dynamical models of crust formation and magmatism as well as planetary thermal evolution models with geological and geophysical observations on the internal structure and surface composition of one-plate planets, the EU-funded CRUSLID project seeks to understand the formation and subsequent evolution of planetary crusts and lithospheres on the Moon, Mars and Venus from a physical perspective.
On a stagnant-lid planet, magma ascent and trajectory strongly depend on the crustal stress state and hence on its surface topography, which is shaped by impact cratering processes. We have shown that a crater topography leads to four distinct regimes of dike propagation, depending on if and where the magma reaches the surface (inside or outside the crater). We use these different behaviors together with systematics in observations of volcanic eruptions or magma intrusions linked to impact craters on the terrestrial planets to interpret these observations in terms of planetary crusts structure. Application to lava-filled dark-floored craters on Venus suggest that the crust forming the crustal plateaus is less than 45 km thick and could be slightly differentiated.
We have identified two mechanisms that can generate long-wavelength variations in the crust and lithosphere structure on a one-plate planet. These long-wavelength variations are caused by instabilities in lithosphere and crustal growth during the last stage of slushy magma ocean solidification, once it has reached the rheological threshold where it behaves as a solid and/or during phases of crustal extraction from a partially molten mantle. They arise because of the pressure-dependence of the solidus of the silicate mantle: (i) the thinner the lithosphere, the less melt in the mantle below, the smaller the mantle viscosity is and the larger the heat flow from the convective mantle is, promoting further thinning of the lithosphere. And (ii) the thicker the crust, the larger its content in heat-producing elements, the hotter the lid and the larger the amount of melt in the mantle below is, promoting more crustal extraction and thickening. Long-wavelength variations in the plate and crustal structure, in particular asymmetric/hemispherical ones, are favored because of the increased effectiveness of thermal conduction at smaller wavelengths. In addition, we have shown that partial crustal differentiation naturally arises in region of thick crusts from the instability in crustal growth, which can explain the formation of evolved rocks in the Martian Highlands.
We have identified two mechanisms that can generate long-wavelength variations in the crust and lithosphere structure on a one-plate planet. These long-wavelength variations are caused by instabilities in lithosphere and crustal growth during the last stage of slushy magma ocean solidification, once it has reached the rheological threshold where it behaves as a solid and/or during phases of crustal extraction from a partially molten mantle. They arise because of the pressure-dependence of the solidus of the silicate mantle: (i) the thinner the lithosphere, the less melt in the mantle below, the smaller the mantle viscosity is and the larger the heat flow from the convective mantle is, promoting further thinning of the lithosphere. And (ii) the thicker the crust, the larger its content in heat-producing elements, the hotter the lid and the larger the amount of melt in the mantle below is, promoting more crustal extraction and thickening. Long-wavelength variations in the plate and crustal structure, in particular asymmetric/hemispherical ones, are favored because of the increased effectiveness of thermal conduction at smaller wavelengths. In addition, we have shown that partial crustal differentiation naturally arises in region of thick crusts from the instability in crustal growth, which can explain the formation of evolved rocks in the Martian Highlands.
We have demonstrated that, on one-plate planet with no plate tectonics, large-wavelengths variations in crustal and lithosphere structure can arise during their early growths if a large fraction of the silicate shell is in the form of a slushy mixture which consists in a solid-liquid mixture with a solid-like rheology. During the second period of the ERC project, we will work to place this result in a comparative planetology perspective as we believe that plate tectonics on Earth could have been driven originally by such large-wavelength variations in its plate structure. We expect that, because these lateral variations in the plate structure take time to develop, they require a non-negligible internal heating, and hence, large enough silicate shells to remain partially molten for a long enough time. They are thus likely to develop on bodies such as Mars and the Moon, but are probably unlikely to grow on Mercury which has a large core and a thinner silicate shell. In addition, we want to explore the effect of the planet size on the expected preferred wavelength for these instabilities.
In parallel, and to further test this slushy mixture model, we will work on an opposite model, where planetary crusts form by crystal flotation on top of a totally liquid magma ocean, and explore whether large lateral variations in the crust structure could be sustained and grow.
As regards to dyke ascent in planetary crusts, we will again work further to place our results in a comparative planetology standpoint. In particular, on smaller gravity planets, craters tend to be deeper for a given radius, which favors horizontal dyke deviation. We thus expect dyke deviation and magma storage to be favored below craters on smaller planets at the expense of eruption on crater floors. We also expect to place constraints on the nature and structure of the crust of the Martian Highlands through the study of Martian floor-fractured craters.
In parallel, and to further test this slushy mixture model, we will work on an opposite model, where planetary crusts form by crystal flotation on top of a totally liquid magma ocean, and explore whether large lateral variations in the crust structure could be sustained and grow.
As regards to dyke ascent in planetary crusts, we will again work further to place our results in a comparative planetology standpoint. In particular, on smaller gravity planets, craters tend to be deeper for a given radius, which favors horizontal dyke deviation. We thus expect dyke deviation and magma storage to be favored below craters on smaller planets at the expense of eruption on crater floors. We also expect to place constraints on the nature and structure of the crust of the Martian Highlands through the study of Martian floor-fractured craters.