A tectonically active early Earth driven by the tidal recession of the Moon
Abstract
How and when Earth developed its dichotomy of felsic continents and mafic seafloor is an enduring mystery. Earth's earliest crust was likely basaltic, but evidence from the earliest rocks and zircons suggests that the first felsic lithologies emerged within ~100 Myrs of the Moon-forming impact. Today, felsic crust forms at subduction zones, but it is unlikely that modern-style subduction occurred early enough to produce the first felsic materials. Various mechanisms have been proposed to form felsic crust in the absence of modern subduction but there is no consensus on their ability to explain the early zircon and rock record.
However, previous studies have neglected an important component of Earth dynamics: rotation. After the Moon-forming giant impact, Earth was rapidly rotating, with a day between ~5 and 2.5 hrs. Earth was significantly oblate, with a ratio of polar to equatorial radii of 0.9 in the canonical model and as low as 0.5 in recently proposed high-angular momentum models. Earth had a very different physical structure (i.e., internal pressure, surface gravity, etc.) than at present. As the Moon receded from Earth, the planet's spin period increased and its shape changed dramatically, becoming roughly spherical within a few 10s Myrs. We used petrological, tidal evolution, and planetary structure calculations to determine the effect of Earth's distorted and changing shape on the early crust. We find that the composition and thickness of a terrestrial crust formed by decompression melting of the primitive mantle varied with latitude due to the varying surface gravity. We demonstrate that the change in shape of Earth caused by lunar tidal recession drove extensive tectonics and deformation of this early crust during the first few tens of millions of years after the Moon-forming giant impact. There would have been extension in polar regions and convergent tectonics in the equatorial regions at rates potentially higher than those forming the Himalayas today. Such substantial deformation could have forced hydrated crust to depth, driving secondary melting and the production of more evolved magmas. A tectonically active early Earth could explain the diversity of lithologies recorded in the early zircon and rock records and would have set the conditions for the subsequent evolution of Earth's surface and interior.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMDI022..01L
- Keywords:
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- 1020 Composition of the continental crust;
- GEOCHEMISTRY;
- 3640 Igneous petrology;
- MINERALOGY AND PETROLOGY;
- 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGY;
- 8157 Plate motions: past;
- TECTONOPHYSICS