Subduction zones located at convergent plate margins are major geodynamic sites that primarily affect plate tectonics and transport of crustal material deep into the Earth’s mantle. Subduction of one tectonic plate below another plate is accompanied by deformation processes enhancing seismicity and by dehydration/ melting of the subducting plate and overlying mantle, which affect magmatism and volatile (including greenhouse gases) recycling into magmas. Major earthquakes and explosive volcanism directly impact thousands of kilometers of coastal and mountain areas located at present-day and fossil convergent margins. However, the mechanisms attending the downwards transport of crustal material during subduction and its tectonic return back to the surface (exhumation) before/during continent collision, are still poorly understood and controversial. This is because the history of subduction cannot be obtained from real-time geophysical or seismic data which only provide static snapshots of subduction zones today. The study of paleo-geological processes is the key to understanding, modelling, predicting and ultimately ameliorating catastrophic geological events impacting people living near convergent margins.
Quantitative understanding the rates and true depths of burial, the thermal regimes of subduction zones and the tectonic processes allied with oceanic subduction and continental collision can only be achieved by determining the pressure-temperature-time-depth (P-T-t-h) histories of Ultra-High-Pressure Metamorphic (UHPM) rocks that have been subducted to pressures greater than 3 GPa and subsequently exhumed to the Earth’s surface.
The fundamental and fascinating challenge is not whether low-density crustal rocks can be subducted to extremely high pressures. Traditionally such pressures have been associated directly with depth h, by assuming that the stress state in the Earth is hydrostatic (so P = ρgh, leading to the simple correlation that 30 km depth = 1 GPa of pressure increase). If pressure in the collision zone environment is uniformly hydrostatic, then the pressures inferred from preserved index minerals indicate that the rocks reached depths of 90, 120 or even 300 km. It is then clear that the fundamental and fascinating problem is not that these dense high-pressure rocks exist. The question is how were they exhumed into the much less dense over-lying crustal units in which they are found?
We aim at using the fossil stresses preserved on inclusions to retrieve the stress state acting on the rocks when the inclusions has been encapsulated. This will enable to determine if the paradigm pressure equal depths is a good approximation or we have to considerably re/think our models for interpreting a large portion of deep geological processes. We will be able to determine for the first time the true depths reached by paleo-subduction processes. This has profound implications on the interpretation of many potentially catastrophic processed from the triggering mechanisms for Earthquakes to the magma genesis and pathways to much more common rock faulting mechanisms.
The project is articulated into three major work packages:
Theoretical development: We will establish the theoretical background for the application of elastic geobarometry on anisotropic mineral phases to enable full use of several phases to determine their P and T of encapsulation.
Validation: We will then validate the theoretical and practical aspects for measurements using laboratory prepared samples
Application to rocks: We will adopt the developed approaches to determine P T and depth of encapsulation for several host inclusion pairs on three terraines (Dora Maira, Lago di Cignana and Western Gneiss Region) selected because their similarities and controversial aspects.
