Pore Space Evolution within Carbonate Fault Rocks

We investigate five extensional carbonate fault zones responsible of destructive earthquakes in peninsular Italy to understand the control exerted by cataclasis and diagenesis on their petrophysical properties. The study fault rocks were exhumed from depths ≤1.5 km during Plio-Quaternary times. They are made up of calcitic and dolomitic survivor grains, carbonate matrix, and possible calcite cements. The grain-supported fault rocks localize within the outermost portions of the fault cores, whereas the matrix-supported in the innermost portions, near the main slip surfaces. By integrating the results of optical microscopy, digital image, ultrasonic, and petrophysical analyses carried out at both room and increasing confining pressures, we are able to assess the pore type, geometry, connectivity and overall network properties in the different fault rock textures. All grain-supported and dolomiterich, matrix supported fault rocks include soft, crack-like pores, which constitute well-connected connected networks. Differently, within the calcite-rich, matrix-supported fault rocks conspicuous precipitation of euhedral calcite that occurred under vadose conditions occluded the largest and well-connected pores, leaving only stiff, sub-spherical pores. The results of this work permit to establish, for the first time, the relative control exerted by deformation mechanisms and diagenetic processes on the pore characteristics in carbonate fault rocks.