The School of Earth and Atmospheric Sciences Presents Dr. Shi "Joyce" Sim, Carnegie Institution for Science

Tectonic Boundaries: A Modeling Perspective for Past and Present

Tectonic boundaries connect Earth’s interior to the surface. I use numerical modeling to study the dynamics at such boundaries where the interconnectedness with life and volatiles is strikingly apparent.

First, using synthetic plate configurations derived from whole mantle convection numerical models appropriate for various periods during Earth’s evolution, I show that mid-ocean ridges have remained submerged and their depths potentially constant through geological times. This allows for maximum hydrothermal activities, which not only influence the mantle’s geochemical evolution but also creates chemical disequilibrium that communities of organisms flourish on.

To further understand dynamics at mid-ocean ridges, I then employ two-phase flow methods to numerically understand how melt focuses from a ~100 km wide region of melt generation to a narrow neo-volcanic area of a few kilometers, varying spreading rates and background mantle permeability. The models can predict oceanic crustal thicknesses versus spreading rates, which fit well with the observations from geophysical surveys.

Three distinct melt focusing mechanisms are recognized in the models: 1) Melting pressure focusing, 2) Decompaction layers and 3) Ridge suction, of which the first two play dominant roles in focusing melt.

The manifestation of these mechanisms depends largely on the rheological parameterization and permeability of the mantle. The location and amount of melt present in the models agree to a certain extent with geophysical observations. The disagreements hint at the possibility that melting pressure focusing could be more significant than that presented here, which could provide constraints to mantle rheology and permeability.