Tectonic uplift is a major contributor to the long term environmental changes. Rivers are a continental leak of nutrients and salts regulating geochemical cycles. These processes occurs in regions of high elevation, above sea-level.
However, I am going to show you that it has not always been the case in the Archean, when continents used to be relatively flat and flooded. This work has been conducted with Patrice Rey and Nicolas Flament from Sydney University.
Indeed, the continental and oceanic crust in the Archean were significantly different than today. And these come from the radiogenic heating of the crust and mantle before 2.5Ga that was, at least, twice that of today.
Archean crust was much and at higher temperature, the rocks composing the crust become softer. As a consequence, they can flow upon the weight of a load or boundary stresses. Because a soft crust si not strong enough to support high elevations, modern-style mountains were forbidden during the Archean.
To test this assertion, we have built a 3D thin sheet model of the Archean lithosphere that we put in a sort of triaxial testing experiment. The column is subject to lateral stresses and we compute the thermal and mechanical response of the lithosphere. In these calculations, we vary the radiogenic heat content and we start from a steady-state geotherm. This assumption can be questionned in such ancient times.
Under these stresses, the lithosphere can thicken and laterally flow depending on its strength. When the Moho temperature exceeds 700°C, the deep crust is too soft and cannot support the mountain load. As a consequence, it flows under its own weight and the maximum plateau elevation that we obtain in our models is around 2000m. However, when the Moho temperature is closer to 500°C, the lithosphere is very strong and resists the gravitationnal force of the mountain building. As you see here, there is a smooth transition between Archean and Proterozoic style of mountain building. From crustal flow dominated to thickenning dominated. Therefore, the Archean hypsometry must have been much flater than today.
If we look at subsidece now, we have the same kind of regulation of the crustal thickness, that has been advocated by Bailey already. Here we show a 2D thermo-mechanical experiment in which we start from an isostatic equilibrium but what you see in black is an emplaced 6km thick flood basalt. The basalt is denser than the surrounding crust and the loading generates lower crustal flow for a hot crust. The time scale of this flow is short as you see here. To sum up: during the Archean the crust is much hotter and lower crustal flow is a dominating tectonic process that regulates crustal thickness very efficienty.
In the Archean the continents were also flooded. In a 1999 paper Nick Arndt showed that a lot of the Archean continental flood basalts were emplaced subaqueaously. Flatenning the continents allow easier flooding, but it is the thicker oceanic crust at that time that had the biggest contribution in flooding the continents.
Indeed, most of the studies tend to show that with a hotter mantle, the oceanic crust was much thicker, reaching probably 20km. Here you have observations of Proterozoic to Archean ophiolites supporting this idea, also developped in models of decompression melting. The crust is the light component in the oceanic lithosphere. Hence, thickenning the oceanic crust changes the isostatic balance. The topographic difference between continents and oceans was then smaller than today.
We computed such isostatic equilibrium, assuming hypsometric and bathymetric models based on our previous work and models of Labrosse and Jaupart (2007). Here is shown the distribution of altitude and sea-level with and Archean type setting.The mantle is 150°C hotter than today, the continents are smaller and flatter. In this calculation, the continents are under 500-800m of water and less than 5% of the surface of the Earth is emerged. Off course, the volume of the oceans may have changed with time and these calculations are indicative.
To have a feeling of what such changes of hypsometry and sea-level could affect the planet, I show you here such changes applied to the present day continental configuration. Most of the world would be under water exceprt the highest elevations generated in tectonic convergent settings.
In conclusion, because the crust was hotter, crustal flow could regulate thickness and elevations preventing modern style mountain building. A hotter mantle would generate a thick buoyant oceanic crust. Taking this into account in the isostatic equilibrium,