Microstructural Evolution of Calcite Fault Gouge in Earthquake-like Experiments

Crookbain, Kieran James (Kieran)

Earthquakes often rupture through carbonate rocks in the upper crust, therefore it is important to understand the microstructures produced during faulting. This may help to identify the mode in which particular carbonate-­‐bearing faults slipped in the past (i.e. seismically during an earthquake or aseismically during creep). This is relevant for earthquake hazard analysis and it is also an important first step in using fault rock microstructures to quantify fault mechanical behaviour (e.g. dynamic stresses, frictional heal anomalies). This study involves a microstructural analysis of calcite fault gouges experimentally deformed under “earthquake-­‐like” conditions (slip velocity >1 m s-­‐1; displacements <5 m; normal stresses < 30 MPa). The specimens were deformed in the Slow to High Velocity Rotary-­‐Shear frictional Apparatus (SHIVA) at the INGV, Rome. Three samples made of pure calcite gouge were analysed in this work. The samples were deformed in a ring-­‐shaped (external/internal diameters of 55 mm and 35 mm) gouge sample holder. Scanning Electron Microscope (SEM) images were collected of the three deformed samples using polished thin sections cut approximately parallel to the slip direction and perpendicular to gouge layer boundaries. Using representative SEM images, grain boundaries were manually traced and the resulting images of the calcite aggregates were quantified (e.g. grain sizes, grain orientations, grain locations) using Image SXM software. Three distinct microstructural domains were identified in the deformed calcite gouges; 1) plastically deformed layers of gouge recrystallization, 2) transitional layers with partial recrystallization and 3) brittly deformed layers. The plastically deformed layer becomes thicker with increasing mechanical work (and frictional heating). Grains within the plastically deformed layer evolve to become more elongate and aligned parallel to gouge layer boundaries with increasing mechanical work. Two well-­‐defined grain shape-­‐ preferred orientations (SPO) are present in the plastically deformed layer. The dominant SPO results from plastic stretching and elongation of grains, while the secondary SPO is due to fracturing of the elongate grains at high angles to their long axes. Grain sizes in the gouge layers generally decrease towards the localized principal slip surfaces, but the data indicate some late stage grain growth in the plastically deformed layers close to principal slip surfaces, possibly reflecting grain annealing immediately following the experimental slip pulses. Overall, the results show that calcite gouge microstructures correlate in a systematic way to both the experimental conditions and the distance from the localized principal slip surface. This is significant because it suggests that experimental and natural microstructures could be reliably compared to understand aspects of the seismic deformation history of natural carbonate-­‐bearing fault zones.

xii, 116 pages A4

2014Crookbain