Shear zones: Natural laboratories of rock deformation and mineral alteration

Robert Sturm (Austria)

During the last few decades, the interest of diverse geosciences has increasingly focussed on the examination of so-called ‘shear zones’, because the displacements between two lithological blocks represent natural ‘laboratories’, within which the phenomena of mineral alteration and deformation are clearly shown for the purposes of scientific study. Many shear zones are only a few centimetres in size, meaning that their examination is relatively easy. Others, like the San Andreas Fault, have a width of several hundreds of metres, which requires a bit more effort to investigate (Fig. 1).

Shear zones – definitions and main characteristics

Broadly speaking, along a shear zone, two lithological units – ranging in size from several square metres to the size of continentals – are displaced against each other. The movement has to be exclusively evaluated in terms of plate tectonics and often represents the cause of earthquakes. Depending on their orientation, three main types of shear zones can be distinguished (Fig. 2):

• The normal fault is characterised by the lowering of a lithological block with respect to its neighbouring tectonic unit. If the face of displacement between the two blocks has only a small angle of inclination, the shearing process is accompanied by crustal extension which is most impressively seen in the Rhinegraben and the East African Rift Valley.
• The reverse fault is commonly marked by the displacement of one lithological unit on or over the other and therefore represents the reverse process to that occurring in the normal fault. Reverse faults are always accompanied by processes of crustal transpression (that is, shortening), which play a major role in the formation of mountain belts.
• The strike-slip fault shows a horizontal displacement between the neighbouring lithological blocks. Such faults may reach lengths of several thousands of kilometres.

The shearing process of two lithological units has certain effects on the mineralogy occurring directly in the contact area. At low temperatures and pressures (that is, shear zones in higher crustal levels), the minerals in the contact zone are mainly subject to ‘brittle deformation’ in which the minerals tend to be broken up – referred to as cataclasis- and fine-grained rocks are produced. At temperatures greater than 400°C and pressures of about 0.5GPa, brittle deformation is successively replaced by ‘ductile deformation’, which often results in the formation of new assemblages of minerals, a phenomenon that is commonly referred to as ‘mylonitization’. The extent of the mylonitization is determined by (among other things) the amount of fluids (for example, H₂O and CO₂) infiltrating the shear zone.

If the shearing process takes place under changing thermal conditions, the contact area of the lithological units is characterised by a combination of brittle and ductile deformation (that is, ‘brittle-ductile’; Fig. 2), resulting in both a crushing of the original crystal assemblage and crystallisation of new minerals. Rocks formed by brittle-ductile deformation often have larger clasts embedded in a matrix of very fine new crystals.

Shear zones under the microscope

Although the macroscopic appearance of shear zones may be very impressive in some cases (Fig. 1), more detailed scientific information can be obtained by their microscopic investigation, requiring the production of thin sections of rock. The questions that can be answered by microscopic work include the type of deformation (see above), the type and extent of mineral alteration, and (no less exciting) the direction of shear. The latter phenomenon describes the direction (sinistral or dextral) along which one geological block is displaced against another. In Fig. 2, all the shear zone sketches show a dextral (right-handed) displacement.

Under the microscope, any determination of the shear is made possible by ‘shear sense indicators’, and a large number of these have been described in the scientific literature. However, within a rock sample, making sense of the occurrence of several shear sense indicators can be a matter of luck. The highest frequency of occurrence can usually be recognised by ‘pressure shadows’, which are accumulations of small, new crystals at the sides of large crystal clasts, which have been rotated away from the shearing pressure (that is, on the lee-sides of the clasts). Some typical pressure shadows of feldspar and garnet are shown in Fig. 3. Other shear sense indicators, with a lower frequency of occurrence, include ‘mica fishes’, which are formed as a result of ductile shear forces acting on sheet silicates, and ‘fractured grain’ structures, which develop because of a shear-induced division of large crystal clasts (Fig. 3).

As already outlined above, ductile shear zones can show extensive mineral alteration in connection with their rocks, due to local increases of pressure and temperature, as well as the infiltration of fluids. The extent of this shear-induced change of mineral assemblages is demonstrated by two shear zones occurring at different places in Austria (Fig. 4).

The first shear zone is exposed in the Central Alps and shows the alteration of a tonalite (a granitic rock type) into a garnet-botite schist, with new crystals of garnet reaching diameters of up to 5cm. The second shear zone is located in the Bohemian Massif and is distinguished by the formation of a cordierite-biotite-feldspar mylonite from a feldspar-biotite-gneiss. While in the first example, alteration processes took place at temperatures of about 550°C and pressures of 0.6 to 0.8 GPa, in the second, the respective temperatures and pressures amounted to 600°C and 0.35 to 0.4GPa. In other words, the formation of the first shear zone happened in intermediate crustal levels (or niveaus) (15 to 20km in depth), whereas the second shear zone was formed at a depth of 10 to 12km.

Finally, it may be concluded that…

Shear zones commonly offer the possibility of studying the process of rock metamorphosis within a very limited area, because most of them only measure several centimetres to a few metres in size. A major advantage of the examination of such small-scale shear zones includes reduced costs and the fact that any results and findings obtained from these small ‘laboratories’ can be applied to large-scale shear zones. In the near future, it is likely that such investigations will help to clarify the connection between shear kinetics and the formation of earthquakes. This will be an essential step towards more reliable predictions of these destructive events.

Further reading

Eisbacher, G. H.: Einführung in die Tektonik. Enke-Verlag, Stuttgart 1991.

Steyrer, H. P. und Sturm, R.: Stability of zircon in a low-grade ultramylonite and its utility for chemical mass balancing: the shear zone at Miéville, Switzerland. Chemical Geology 187, 1–19 (2002).

Sturm, R.: Gesteinsmetamorphose unter dem Mikroskop – Mineralumwandlungsprozesse bei abnehmenden Druck- und Temperaturbedingungen. Mikrokosmos 97, 267–271 (2008).

Sturm, R.: Im Mineral eingesperrt – Mikroskopie von Einschlussphasen in magmatischen Kristallen. Mikrokosmos 98, 283–289 (2009).