GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 9-2
Presentation Time: 8:45 AM


STERN, Robert J., Geosciences, U TX Dallas, Geosciences Dept. ROC 2.1, 800 W , Campbell Ave., Richardson, TX 75080 and GERYA, Taras, Institute of Geophysics, ETH Zentrum, Sonneggstrasse 5, Zürich, 8092, Switzerland

Most active silicate bodies >1000 km diameter (planetoids) have magmatotectonic regimes that can be usefully characterized as “single lid” (SL). SL planetoids show a wide range of deformation and magmatic styles, from intense (Io) to vigorous (Venus) to geriatric (Mars); once tectonic and magmatic activity ceases, a dead SL regime (Moon, Mercury) ensues. Earth today is unusual in having plate tectonics (PT) but it is controversial when in the past Earth experienced PT vs. active SL. Resolving Earth’s tectonic history (PT vs. SL) is important for understanding biological evolution. Once life begins on an active planetoid with continents and oceans, evolution on a SL body will proceed more slowly than on a similar body with PT. Faster biological evolution on PT planetoid is likely for three reasons: 1) higher sealevel and greater areas of continental shelves; 2) enhanced nutrient supply; and 3) higher rates of isolation and recombination. Higher sealevel (and enhanced continental flooding) occurs with PT because seafloor spreading continuously creates new seafloor, which is shallower than old seafloor. Large expanses of young seafloor around spreading ridges displaces water onto the continents, resulting in broader continental shelves which in turn maximizes opportunities for early animals and plants to evolve. SL planetoids are dominated by old, deep seafloor, so sealevel, the area of continental shelves, and the opportunities for evolution are reduced. Secondly, life and evolution depend on nutrients such as Fe, N and P and the supply of these will be greater on a PT than on a SL planetoid. This is because erosion rates are higher with PT than SL as a result of continental collisions and because of greater seafloor hydrothermal activity associated with seafloor spreading, both of which release nutrients to oceans and soils. Finally, evolution rates increase with isolation, which allows speciation, followed by recombination and competition. Isolation and recombination occurs as a result of continental rifting and collision, which happens continuously on a PT planetoid but not on a SL body. Evolution rates on a planetoid with life will evolve slowly when it is in SL phase and rapidly when it is in PT phase. Correspondingly, when a SL planetoid with life transitions to PT, evolution rates will increase significantly.