The Appearance of Highly Reflective Fault Mirrors in Carbonate Rocks
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The appearance of highly reflective fault mirrors (FMs) (Figure 1) in carbonate rocks is a topic that is only recently receiving interest. Knowing the conditions that are required to produce these FMs is important as it can indicate how the fault has ruptured providing a mitigation tool for appropriate plans to be put in place for similar events in limestone dominated regions. This is geologically important, has economical significance and could save lives. Recognised FMs occur in carbonate rocks during presumed faulting which is a common occurrence in the earth’s upper crust (Barnhoon et al 2005), especially through Mediterranean countries such as Greece and Italy (Smith et al 2011). Fault mirrors have previously been studied at larger outcrop scale, but the nanoscale study has been much overlooked.
Figure 1. Highly reflective FM in Eocene
Limestone, Kfar Giladi, Israel (Siman-Tov et al)
The first major nanoscale study was undertaken using hand samples of carbonate FMs from three different well-preserved locations along the Dead Sea transform. They were compared with a non-mirror like fault surface from the Nahal Uziyahu Fault, Gulf of Eilat as a control. The surface topography of the samples were scanned down to the submicron scale using an optical profilometer as well as using atomic force microscopy (AFM), both which are non-contact instruments that are used to map surface profiles by using the reflections of a laser beam to accurately map the surface of a sample. The optical profilometer is quicker but does not have as high resolution as the AFM. Scanning electron microscopy (SEM) was used to image the surface structures and transmission electron microscopy was used to study a cross sectional area of the gathered samples (Siman-Tov et al 2013).
Structures and smoothness of the FMs vary depending on the scale (Siman-Tov). A ~10-2 m layer of 90-100% matrix called ultracataclasite is observed (Sibson 1977), and the surface has corrugations with wavelengths ranging from ~ 0.1 – 1.0 m in the larger outcrop scale overview of the surface. Between hand sample scale and ~1 μm, most the observed surfaces showed a few subparallel striations but were mostly smooth. Striations are common on slip surfaces (Siman-Tov). Roughness decreases with increasing slip, which suggests the area has been subjected to large slip amounts. However the occurrence of extensional fractures known as Riedel shears which branch off the striations suggest that the area was exposed to varying amount of slip. When analysed at in the nanoscale, carbonate grains with diameters of tens to hundreds of nanometers are visible (nanograins).