Origins of seismic reflectivity in lower crustal rocks

At the beginning of the 20th century through the study of earthquake waves, Andrija Mohorovicic discovered the boundary between the Earth`s crust and the Mantle, now called "Moho" in his honour. For seismologists this boundary is very striking because it reflects and refracts seismic waves, however, its physical significance has been left to speculation for a long time. This is because it lies at an average depth of 30 km below the continental surface, at a practically inaccessible depth. The pressures and temperatures are so high beyond a depth of 10 km that even the most modern drilling materials fail. However, mineralogists can simulate such conditions in their laboratories to derive conceptual models of the rocks of the deep crust and upper Mantle. In addition, there are clues about the composition of these rocks from rare finds of highly weathered rocks that have been geologically squeezed out from deep inside the Earth. However, despite these finds, modern knowledge about the composition of the "Moho" is essentially of indirect nature. We want to change that with this research project. For this purpose, together with colleagues from Italy, Switzerland and the USA, we have chosen a site in the Italian Western Alps where the "Moho" has been uplifted to a depth of less than 5 km by tectonic mountain-building processes. Typical rocks of the deep crust are present here only a few hundred metres from the surface and in a relatively unweathered condition. We are using this globally unique situation to perform a joint drilling project that aims to drill through the "Moho". In the first phase of the project, we want to explore the rocks of the deep crust from two 1 km deep boreholes, to better understand how their seismic properties are related to their physical composition. To do this, we will carry out numerous geophysical measurements in the boreholes, combined with seismic experiments on the surface, which we will then evaluate and link together using modern imaging techniques and artificial intelligence methods. We expect that the insights from this project will be used worldwide to better understand the existing seismic images of the "Moho" and their significance to dynamics of the Earth.