The characterization of frictional properties and deformation mechanisms of carbonate–bearing normal faults is fundamental to characterize reservoir development and seismic hazard. We conducted 24 experiments on simulated calcite fault gouges made of powdered Carrara Marble. Experiments have been performed at room temperature with a biaxial apparatus in double direct shear configuration, using CaCO3-equilibrated water as saturating fluid, applying constant normal stress (n) in the range 5-120 MPa and constant sliding velocity (between 0.3 µm/s and 100 µm/s). Deformed gouge layers have been observed at optical and SEM microscope. Our results show that simulated calcite fault gouges display a transition from brittle to semi-brittle behavior passing from low (n 20 MPa) to high (n > 20 MPa) normal stresses. At low normal stresses shear strength linearly increases with normal stress with no effect of sliding velocity. At higher stresses shear strength increases non-linearly with normal stress with the greater effects at slow sliding velocities, causing a roll-over of the Mohr-Coulomb failure envelope. The collected microstructures reflect the two different mechanical behaviors. Microstructures related to experiments conducted in brittle conditions do not change with sliding velocity and exhibit only cataclastic processes with grain comminution concentrated in slip localization zones. On the other hand, semi-brittle microstructures exhibit foliation in gouge portions characterized by high grain comminution, suggesting cataclasis plus pressure solution accommodated slip. Moreover, microstructures related to experiments conducted in semi-brittle conditions vary with slip velocity and slip localization zones related to semi-brittle experiments are composed of sub-parallel shear bands constituted by rounded, 10-100 nm sized, nanoparticles. Mechanical and microstructural observations suggest that the brittle to semibrittle transition is achieved when slip is accommodated by cataclasis accompanied by some amount of pressure solution, that increases both with increasing normal stress, and decreasing slip velocity. The onset of semi-brittle behavior at relatively low values of normal stress (about 20 MPa) suggest that carbonate-bearing faults may be prone to semi-brittle creep at the shallow crustal conditions (1-10 km depth).
The semibrittle behavior of simulated calcite fault gouge / Mercuri, Marco; Scuderi, MARCO MARIA; Tesei, Telemaco; Collettini, Cristiano. - (2017). (Intervento presentato al convegno 12th EUROconference on Rock Physics and Geomechanics tenutosi a Jerusalem, Israel nel 5-10 novembre 2017).
The semibrittle behavior of simulated calcite fault gouge
Marco Mercuri
Investigation
;Marco ScuderiMembro del Collaboration Group
;Cristiano CollettiniSupervision
2017
Abstract
The characterization of frictional properties and deformation mechanisms of carbonate–bearing normal faults is fundamental to characterize reservoir development and seismic hazard. We conducted 24 experiments on simulated calcite fault gouges made of powdered Carrara Marble. Experiments have been performed at room temperature with a biaxial apparatus in double direct shear configuration, using CaCO3-equilibrated water as saturating fluid, applying constant normal stress (n) in the range 5-120 MPa and constant sliding velocity (between 0.3 µm/s and 100 µm/s). Deformed gouge layers have been observed at optical and SEM microscope. Our results show that simulated calcite fault gouges display a transition from brittle to semi-brittle behavior passing from low (n 20 MPa) to high (n > 20 MPa) normal stresses. At low normal stresses shear strength linearly increases with normal stress with no effect of sliding velocity. At higher stresses shear strength increases non-linearly with normal stress with the greater effects at slow sliding velocities, causing a roll-over of the Mohr-Coulomb failure envelope. The collected microstructures reflect the two different mechanical behaviors. Microstructures related to experiments conducted in brittle conditions do not change with sliding velocity and exhibit only cataclastic processes with grain comminution concentrated in slip localization zones. On the other hand, semi-brittle microstructures exhibit foliation in gouge portions characterized by high grain comminution, suggesting cataclasis plus pressure solution accommodated slip. Moreover, microstructures related to experiments conducted in semi-brittle conditions vary with slip velocity and slip localization zones related to semi-brittle experiments are composed of sub-parallel shear bands constituted by rounded, 10-100 nm sized, nanoparticles. Mechanical and microstructural observations suggest that the brittle to semibrittle transition is achieved when slip is accommodated by cataclasis accompanied by some amount of pressure solution, that increases both with increasing normal stress, and decreasing slip velocity. The onset of semi-brittle behavior at relatively low values of normal stress (about 20 MPa) suggest that carbonate-bearing faults may be prone to semi-brittle creep at the shallow crustal conditions (1-10 km depth).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.