Application of Non-Extensive Statistical Physics on the particle size distribution in natural carbonate fault rocks

Abstract

This contribution focuses on the results of combined generalized statistical mechanics and microstructural analyses of carbonate fault rock samples, which derive from active extensional fault zones cropping out in peninsular Italy. Specifically, this work is aimed at deciphering the role played by both pre-existing structural elements and structural diagenesis on the dimension and distribution of the largest survivor grains. The studied extensional fault zones were exhumed from shallow crustal depths during the Plio-Quaternary downfaulting of the Apennines. They crosscut Mesozoic platform limestones and dolostones. Despite the different host rocks, all carbonate fault rocks exhibit very similar cataclastic textures but dissimilar pore characteristics. In this work, we apply for the first time the principles of the Non-Extensive Statistical Physics to the study of fault rocks. By considering the Tsallis entropy, which is expressed by the Q-value representing the hierarchical organization of the system, we consider the crossover point value, V0, computed for both 1cm2 and 9cm2 representative fault rock images at the transition from Q-exponential to exponential behaviors. As a result, we document that the limestone grain-supported rocks, which mainly localize in the outer fault cores, are always characterized by smaller V0 values with respect to the dolostone ones. These data are interpreted as due to both burial and thrusting-related pressure solution seams, which pervasively formed in the limestone host rocks. Conversely, the amount of V0 variations computed from outer to inner fault cores are associated to the effects of widespread calcite cement precipitation, which mainly occurred in the limestone matrix-supported fault rocks. By considering the weighted, average areas of survivor grains with dimension larger than V0 a as a proxy for sorting, we document an increase from outer to inner fault cores in all the study fault zones. At a close view, all dolomite-rich fault rock samples show very similar median values, whereas the matrix-supported fault rock samples of the inner limestone fault cores are characterized by smaller median values relative to the grain-supported of the limestone outer fault cores. This discrepancy is interpreted as also due to widespread cementation processes, which formed aggregate grains in the fine-grained fault rocks encompassing the main slip surfaces of the limestone fault cores.