Eclogitization

Thickening of continental crust can cause high-pressure metamorphic reactions produce eclogite. In continental crust, eclogite is the rock that forms from the break down of plagioclase and orthopyroxene and growth of garnet and clinopyroxene in their place. Certain forms of garnet and clinopyroxene are much more dense than the parent rock. The primary result of large-scale crustal eclogitization, then, is a density increase that reduces the buoyancy of the affected part of the crust.

Our research suggests that the lower crust can even become negatively buoyant with respect to underlying mantle. In a recent study, we constrain the range of compositions which attain this negative buoyancy and find that they include rocks with less than 54% silica (by weight). If such crust is weak enough—for example, if it is situated above a hot mantle wedge in a back-arc orogen—the base of the crust can founder gravitationally over 10-Myr timescales. Crustal foundering can affect important geological processes such as mountain building and sedimentary basin development. But the tectonic conditions that lead to eclogitization and foundering of the lower crust over relevant timescales are not currently well understood, partly because the coupled thermodynamic-geodynamic system is difficult to model.

A major part of my current research consists of modelling how this process operates using thermodynamic and geodynamic simulations. The goal of this research is to understand how and where foundering may have occurred on Earth, and then to better constrain the predicted consequences in the upper crust and geologic record.