NANOGRAINS FORM CARBONATE FAULT MIRRORS

Many faults are characterized by naturally polished glossy surfaces, termed fault mirrors (FMs), which form during slip. The structure of FMs and the mechanism of their formation are important for understanding the mechanics of frictional sliding in general, and during earthquakes in particular.

We characterized the small-scale structure of natural carbonate FMs from 3 different faults along a tectonically active region of the Dead Sea transform. Atomic force microscopy measurements indicate that the FMs possess extremely smooth surface topography, accounting for their mirror-like appearance. Electron microscope characterization tools revealed a thin (< 1 µm) layer, composed of tightly packed nano-scaled grains, coating a rougher layer composed of micron-size calcite crystals. The crystals contain closely-spaced, plastically-formed, mechanical twins, which define new sub-grain boundaries. The narrow sub-grains are observed to buckle and break into sub-micron pieces near the sheared surface. This observation suggests a new brittle-ductile mechanism for nano-grain (NG) formation. In addition, observations of rounded NGs and lack of scratches on the submicron scale, suggest that ductility may control deformation within the NG layer itself. The role of ductility during frictional sliding, both in forming the NG layer, and in the deformation process of the powder, may be critical for understanding shear on geological faults.

To better understand the field results, we also performed high velocity shear experiments. These are in agreement with previous studies, suggesting that FMs form under low normal stress (<30 MPa), due to rapid sliding between rock surfaces (>0.1 m/s), coinciding with pronounced friction reduction. The experimental results, together with our observations on field samples, may suggest a new indictor to identify paleo-seismic slip in shallow burial depth carbonate faults.