Abstract
Abstract. Calcite and dolomite are the two most common minerals in carbonate-bearing faults and shear zones. Motivated by observations of exhumed seismogenic faults in the Italian Central Apennines, we used a rotary-shear apparatus to investigate the frictional and microstructural evolution of ca. 3 mm thick gouge layers consisting of 50 wt % calcite and 50 wt % dolomite. The gouges were sheared at a range of slip rates (30 µm s−1–1 m s−1), displacements (0.05–0.4 m), and a normal load of 17.5 MPa under both room-humidity and water-dampened conditions. The frictional behaviour and microstructural evolution of the gouges were strongly influenced by the presence of water. At room humidity, slip strengthening was observed up to slip rates of 0.01 m s−1, which was associated with gouge dilation and the development of a 500–900 µm wide slip zone cut by Y-, R-, and R1-shear bands. Above a slip rate of 0.1 m s−1, dynamic weakening accompanied the development of a localised < 100 µm thick principal slip zone preserving microstructural evidence for calcite recrystallisation and dolomite decarbonation, while the bulk gouges developed a well-defined foliation consisting of organised domains of heavily fractured calcite and dolomite. In water-dampened conditions, evidence of gouge fluidisation within a fine-grained principal slip zone was observed at a range of slip rates from 30 µm s−1 to 0.1 m s−1, suggesting that caution is needed when relating fluidisation textures to seismic slip in natural fault zones. Dynamic weakening in water-dampened conditions was observed at 1 m s−1, where the principal slip zone was characterised by patches of recrystallised calcite. However, local fragmentation and reworking of recrystallised calcite suggests a cyclic process involving formation and destruction of a heterogeneous slip zone. Our microstructural data show that development of well-defined gouge foliation under the tested experimental conditions is limited to high velocities (>0.1 m s−1) and room humidity, supporting the notion that some foliated gouges and cataclasites may form during seismic slip in natural carbonate-bearing faults.
Highlights
Calcite and dolomite are the most common minerals in carbonate-bearing faults and shear zones (e.g. Busch and van der Pluijm, 1995; Snoke et al, 1998; Bestmann et al, 2000; De Paola et al, 2006; Molli et al, 2010; Tesei et al, 2014; Fondriest et al, 2015, 2020; Delle Piane et al, 2017)
Brittle failure of dolomite grains eventually allowed the calcite-rich layers to become continuous and to continue deforming by superplastic flow (Kushnir et al, 2015). These torsion experiments were performed under significantly elevated temperatures and pressures compared to the experiments in this paper, the results suggest that the occurrence of dolomite in calcite aggregates can influence the mechanical behaviour during deformation
Significant dynamic weakening was observed at a slip rate of 1 m s−1, following a short initial strengthening phase that was followed by a steady state of μss of 0.28 (Figs. 2a, 3b)
Summary
Calcite and dolomite are the most common minerals in carbonate-bearing faults and shear zones (e.g. Busch and van der Pluijm, 1995; Snoke et al, 1998; Bestmann et al, 2000; De Paola et al, 2006; Molli et al, 2010; Tesei et al, 2014; Fondriest et al, 2015, 2020; Delle Piane et al, 2017). The rheology, deformation mechanisms, and frictional behaviour of calcite and dolomite show important differences. 700 ◦C (Kushnir et al, 2015), while calcite can undergo recrystallisation at temperatures as low as 150–200 ◦C (Kennedy and White, 2001) This pronounced difference in deformation style under similar ambient conditions may significantly influence the rheology of faults and shear zones in which the two phases co-exist (Oesterling et al, 2007; Kushnir et al, 2015)
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