The Appearance of Highly Reflective Fault Mirrors in Carbonate Rocks

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).

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"The Appearance of Highly Reflective Fault Mirrors in Carbonate Rocks." 29 Mar 2020

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Surface roughness analysis showed that the nanograins act as a coating over a rougher layer of larger micron-size calcite crystals. The surfaces are smoother than they anticipated due to the deferentially stratified surface profile that was observed in previous larger scale studies (Renard et al 2006 and Candela et al 2009). This was identified using AFM.
The polishing industry uses three stages to create a reflective mirror like polish. These three stages are consistent with the observations from the carbonate FMs and experimentally deformed samples (Adachi and Kato 2000). This begins with the reduction of surface roughness through brittle abrasion and the formations of striations. Nanograin recrystallization then occurs through the reduction of size of the grains, and finally smearing of the nanograins forms the smooth hard surfaces on the nanograins.
The reason that these surfaces reflect light is due to the roughness and can be explained root mean square (RMS) mathematical methods. The three FM samples and control were compared. Light rays with wavelength of 550 nm were reflected off the surfaces. The FMs showed RMS values of <20 nm while the control was in contrast with >100 nm (Siman-Tov).
It is difficult to understand why and how nanograins form, even though they have been observed in both natural and experimental situations. This is because fragmentation processes that lead to them is not expected to form grains less than ~1μm in size. However, instead of the brittle cracks, plastic deformation is dominant in the three samples. Nanograins are formed in the milling industry, and studies into this show that they occur in both high and low strain rates and do not need high temperature environments; it is thought that in the FMs the nanograin layer also has this relationship (Siman-Tov).
Twinning is observed in the calcite nanograins. Unlike most minerals, twinning occurs in calcite at both high and low strain rates (Han et al 2011). This has lead to the theory that the nanograins are deforms in both plastic and brittle manners (Siman-Tov).
It is apparent that ductile deformation occurs within the nanograin layer. It is though that because the scratches that are observed in hand samples have greater size than 1μm they disappear when looking at the sample in the submicron scale, as they are too large to be observed. This is explained by the grind limit, where grains <1 μm deform plastically and do not directly scratch the surface. The nanograins are mostly rounded. This can only be explained as plastic or ductile deformation because fracturing does not occur in the sub-micron scale (Siman-Tov).
Experimental samples identified that there is a relationship between FMs and frictional weakening. This can be observed in natural FM samples. It is not well understood but it shows that they can form in high slip rates.
It is still not completely clear that these FMs have been caused through seismic or aseismic conditions, however, the combinations of on going analysis of natural and in earthquake like test experimental samples and the that they all form in seismically active regions suggest that they form seismically. The exact conditions for the formation of the FMs are still not known and research into this is still ongoing. Research is favouring to the idea that seismic slip has resulted in their formation. The large evidence that supports this is location. All of the FMs observed have been in the tectonically active Dead Sea transform, which since the Miocene has undergone 100 km of offset (Siman-Tov). Recent torsion and shear experiments preformed, suggest that the formation of FMs only occurs during high shear rates (Smith et al 2013).

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