Haakon Fossen and Geane Carolina G. Cavalcante published their seminal review on the topic of shear zones in 2017 in the journal Earth-Science Reviews. The work summarizes the scientific findings on the development and quantification of ductile shear zones sincefrom the the 1960s up to recently. It describes the fundamental aspects of the initiation and growth of shear zones across scales. First, Fossen and Cavalcante summarize the influence of simple shear and pure shear on shear zone development, including the mathematical foundation to quantify the degree of coaxiality, i.e., the role of rotation of the principle deformation axes. They further stress that in natural systems shear zones typically are not plane-strain, and extend their considerations to three dimensions.
After establishing the basis for the description of shear zones, Fossen and Cavalcante focus on reviewing the state-of-the-art (current understanding) on the nucleation and growth of shear zones. After on many years of research, they show that pre-existing flaws or heterogeneities are necessary for the nucleation of shear zones. These flaws range from randomly distributed weaker mineral phases, to planar discontinuities such as fractures or dykes (Fig. 1). Subsequently, shear zones grow. In this context, shear zones that accommodate more displacement are, in general, longer and wider. However, the authors point out that there is significant scatter in this data and the relationship is not straight-forward. Particularly shear zone widening is somewhat enigmatic as many observations from natural occurrences would suggest that shear zones soften with strain, which should lead to them narrowing or at least maintaining their width.
With this in mind, the remainder of this Must-read paper is dedicated to discussing several methods to determine the kinematic vorticity number (Wk) – a measure of the coaxiality of shear zones – from natural occurrences. Several methods can be used, including but not limited to the angle of the foliation, porphyroclast systems, and crystallographic data. In discussing these methods, the authors point out that each of them illuminates a different aspect of shear zone evolution, and that they thus may sample different stages during the deformation history, and the combination of methods may therefore lead to a more complete picture. Simultaneously, Fossen and Cavalcante show that care must be taken in applying these methods, as all of them require some assumptions to be made and that further observations from natural shear zones together with systematic numerical modeling is required to better understand how ductile shear zones accommodate deformation.
As such, this Must-read paper provides a comprehensive and detailed review of the current state of knowledge concerning ductile shear zones and has quickly become one of the go-to publications for much of the following literature. The authors provide guidance and recommendations on how to approach studies dealing with ductile shear zones for anyone that wants to dive deeper into this important topic.
Fig.1. Paired shear zones on the boundary between an aplite dyke and the host granodiorite from the Neves area, Tauern Window, Alps. The occurrences of small-scale shear zones in this area is iconic and has been instrumental in understanding the role of precursors on shear zone nucleation (e.g., Pennacchioni and Mancktelow, 2007). Picture taken by Sascha Zertani.
Sascha Zertani and the TS Must Read Team
References:
Fossen, H., Cavalcante, G.C. (2017). Shear zones – A review. Earth-Science Reviews, 171, 434-455. https://doi.org/10.1016/j.earscirev.2017.05.002
Pennacchioni, G., Mancktelow, N.S. (2007). Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions. 29(11), 1757-1780. https://doi.org/10.1016/j.jsg.2007.06.002
