Tectonic and magmatic processes during extension in planetary lithospheres : rifting, spreading, delamination, and recycling on Earth and Venus
The contrast between Earth and Venus exemplifies how similar planets can dramatically diverge in their evolutionary pathways and highlights the importance of developing a theoretical framework for the origins of planetary diversity. A key aspect of planetary tectonic systems is how the lithosphere responds to driving forces such as the upwelling or downwelling of material in the mantle, especially for extensional systems. On Earth, the processes underpinning continental break-up are poorly understood despite decades of study. In this dissertation, I present a synthesis of numerical modeling, high resolution seismic experiments, and ophiolite geology to reveal how the mantle lithosphere controls the transition from continental rifting to seafloor spreading. The study finds that the transition is controlled by high temperature shear zones in the mantle and by a change in stress from gravitational collapse to buoyant upwelling of a melt-rich mantle. On Venus, extensional systems form a global network across the planet, superficially similar to seafloor spreading on Earth. Using the same numerical modeling methods, I present work showing that the global rift network on Venus is not consistent with the systemic formation of new crust. The exception to this are certain coronae along Venus’ rifts that may represent localized recycling of the crust driven by the delamination of eclogite roots at the base of the crust. Finally, we test that hypothesis for rift-embedded coronae formation and link their formation to global geodynamics, highlighting that Venus’ regional tectonics is highly sensitive to thermal conditions in the lithosphere and mantle. All of this shows the importance of understanding regional tectonics and local lithosphere dynamics, since they can alter or even overprint the expected signal from global geodynamics. The contrast between Earth and Venus also suggests that extension on Earth is dominated by slab-pull, slab-suction, and other far-field stresses associated with downwelling while Venus is dominated by mantle upwelling, despite regional variations. Because the behavior of Venus’ lithosphere is primarily a result of an elevated geothermal profile, this indicates that climate-tectonic coupling is the engine for terrestrial planetary diversity.
Earth, Venus, Corona, Numerical modeling, Rifting, Crustal recycling, Delamination
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