A combination of petrological and joint chemical- mechanical inversion approaches to unravel deep geodynamic processes

Abstract

The present thesis demonstrates the significance of integrating petrological data with numerical models that address diffusion processes and the mechanical behavior of key petrogenetic minerals. Specifically, the metamorphic rocks found in the metamorphic sole of the Pindos ophiolite (northwestern Greece) are investigated to elucidate the geodynamic processes responsible for their formation. Metamorphic soles are key petrotectonic units within ophiolite sequences that preserve valuable information on the emplacement of ophiolitic rocks onto continental crust. To gain a deeper understanding on the formation of the Pindos metamorphic sole, the thesis is split into three main chapters (Chapters 2, 3 and 4). Following an introductory chapter, Chapter 2 presents the petrological data acquired from the metamorphic sole rocks. Multiple thermobarometric methods were employed in combination with Ar-Ar and U-Pb geochronology, alongside a detailed petrographic and compositional analysis. These data provided insights into the peak metamorphic conditions experienced by the sole rocks and offer useful constraints on their subsequent cooling and retrograde history. In Chapter 3, the data from Chapter 2 are used to perform inverse diffusion modelling. Forward diffusion models of major elements in garnet and argon in muscovite are applied to quantify key parameters, such as the cooling rate and the initial equilibration temperature, that reproduce the observed data in the Pindos metamorphic sole. The diffusion models are integrated with a Hamiltonian Monte Carlo approach to efficiently explore the parameter space. Since this approach was used in petrological inversion for the first time, Chapter 3 also includes a detailed explanation of the underlying equations and theory of Hamiltonian Monte Carlo method. Using the results of Chapters 2 and 3, Chapter 4 presents the results of a one-dimensional thermomechanical model that simulates the deformational behavior of quartz inclusions in garnet. The (forward) mechanical combined with Hamiltonian Monte Carlo is used to quantify the cooling and decompression rates as well as the initial temperature and pressure conditions responsible for the currently observed residual pressure of quartz in garnet. An important finding of the present thesis is the profound agreement between all inverse methods employed. The combined insights from these approaches indicate that a transient process was responsible for the formation of the Pindos metamorphic sole rocks. In particular, this process is interpreted to be the fast quenching of a small heat source, most probably the quenching of heat producing shear zone. This thesis emphasizes the importance of integrating conventional petrology with both chemical and mechanical modeling to gain insights into the dynamics of metamorphic processes.