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Crust formation and deformation on the rapidly rotating early Earth

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The last event in the main stage of Earth’s accretion is thought to be the Moon-forming giant impact. A planet-sized body slammed into the proto-Earth, injecting material into orbit out of which the Moon formed. The huge torques exerted during the impact meant that the early Earth was rapidly rotating, with a day between ~ 5 and 2.5 hrs. As a result, Earth was significantly oblate, with a ratio of polar to equatorial radii between 0.9 and 0.5, with a very different physical structure (e.g., internal pressure, surface gravity) 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. This is a key period of Earth’s history in which the first crust formed, the initial atmospheric composition was set, and the conditions for Earth’s subsequent evolution established. However, little work has been done to understand the role that Earth’s rapid rotation played during this epoch.

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 deformation of this early crust during the first few 10s Myr after the 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.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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