Deformation mechanisms active during “brittle” faulting in carbonate gouges

Seminar by Matteo Demurtas

Carbonate-built rocks are a widely recurrent lithology often involved in most of the present seismicity occurring in densely populated areas, such as the circum-Mediterranean area (e.g., L’Aquila 2009, M6.1; Norcia 2016, M6.5). The investigation of the deformation processes that are active during faulting is essential for a more sound understanding of strain accommodation mechanisms during the seismic cycle.The aim of this talk will be focused on the investigation of the deformation mechanisms active during sub- to coseismic sliding (i.e., slip rates from 30 µm/s up to 1 m/s) in experimentally deformed carbonate gouges by means of electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction (TKD) analysis. Microstructural analysis shows the formation of a foliation in the gouge only during coseismic sliding, suggesting this peculiar microstructure to be a potential indicator for seismic slip in the rock record. EBSD analysis on the experimentally deformed gouges shows the development of a crystallographic preferred orientation (CPO) in calcite in the bulk gouge during low-temperature deformation. The CPO is interpreted as a result of grain rotation and preferential fracturing along the {1014} calcite cleavage planes during the early stages of cataclasis. The TKD analysis was focused on the highly-localized principal slip zone of calcite-dolomite mixtures sheared at coseismic slip rates and revealed a recrystallized nanograins aggregate. Here, coseismic slip was likely accommodated mainly by grain-boundary sliding aided by diffusion creep and secondarily by dislocation creep.All the observations from the field- to the nanoscale suggest that earthquakes occurring in the shallow crust made of carbonate-built rocks are likely the result of a combination of elasto-frictional ("brittle") but especially viscous-plastic ("ductile") micro-processes.

Published Nov. 3, 2018 4:15 PM - Last modified Nov. 3, 2018 4:15 PM