Earth-Science Reviews 171 (2017) 434–455 Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Shear zones – A review MARK ⁎ Haakon Fossena,b, , Geane Carolina G. Cavalcantec a Department of Earth Science/Museum of Natural History, University of Bergen, Allégaten 41, N-5007 Bergen, Norway b Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, Cidade Universitária, São Paulo, SP CEP 05508-080, Brazil c Departmento de Geologia, Universidade Federal do Paraná, Av. Cel. Francisco Heráclito dos Santos, s/n, Centro Politécnico, 81531-980, Curitiba, PR CEP 81531-980, Brazil ARTICLE INFO ABSTRACT Keywords: Strain in the lithosphere localizes into tabular zones known as shear zones that grow from small outcrop-size Shear zones individual zones to large composite structures. Nucleation is related to distributed microscale flaws or mesoscale Kinematics structures such as fractures and dikes, and they soon establish displacement profiles similar to faults. Also similar Strain localization to faults, they grow in width and length primarily by segment linkage as they accumulate strain and displace- Vorticity ment, and this process typically results in shear zone networks. Consequently, mature shear zones are hetero- Crustal deformation geneous and composite zones characterized by anastomosing patterns and local variations in thickness and finite strain. Kinematic vorticity estimates suggest that most shear zones deviate from simple shear, and even if subsimple shear may be a useful reference model in many cases, finite strain data indicate that many shear zones involve three-dimensional combinations of coaxial and non-coaxial deformation, such as transpression and transtension. Strain geometry and kinematic vorticity can vary significantly within shear zone networks, which makes it difficult to estimate the bulk deformation type for a composite shear zone or shear zone network. However, perhaps the most challenging aspect is that of progressive deformation, i.e. to what extent and how flow parameters change during deformation (non-steady state deformation), which needs to be addressed by a combination of detailed field observations and numerical modeling. 1. Introduction during rifting (Powell and Glendinning, 1990; Butler et al., 2008; Bird et al., 2015; Phillips et al., 2016) and are important components in the The majority of strain accumulated in the mostly plastic or viscous context of plate tectonics (Bercovici and Ricard, 2012). part of the lithosphere, both in the crust and the mantle (e.g., Vauchez High-strain zones have been recognized in naturally deformed rocks et al., 2012; Snyder and Kjarsgaard, 2013), localizes into zones that since the 19th century (e.g., Reusch, 1888), and particularly since the show large variations in orientation, length, thickness, displacement, theory of thrusting was introduced (e.g., Bertrand, 1884; Geike, 1884; strain geometry, coaxiality, and deformation mechanisms. Such zones Törnebohm, 1888; Peach et al., 1907). However, even though strain in typically involve a significant component of simple shear, and are deformed rocks was discussed relatively early on (e.g., Harker, 1885), therefore called shear zones (Ramsay and Graham, 1970; Sibson, 1977; sound and quantitative analysis of shear zones in terms of geometry, Simpson and De Paor, 1993; Ramsay, 1980), although a component of strain and kinematics founded in mathematical analysis is relatively coaxial deformation (e.g., pure shear) is also commonly involved new, and basic aspects of such analyses were presented in a systematic (Ramberg, 1975a,b; Coward and Kim, 1981; Fossen and Tikoff, 1993; way by Ramsay (1967) and in a series of papers in the 1970s and1980s, Northrup, 1996). Shear zones separate less strained or unstrained notably Ramsay and Graham (1970), Ramberg (1975a,b), Coward portions of the lithosphere, and are the deeper counterparts to upper (1976), Cobbold (1977a,b), Berthé et al. (1979), Lister and Williams crustal faults and fault zones in contractional (thrust), extensional and (1979), Mandl et al. (1977), Sibson (1977), Ramsay (1980), Cobbold strike-slip settings alike (e.g., Sibson, 1977; Scholz, 1988; Wernicke, and Quinquis (1980), and Lister and Snoke (1984). The typical ap- 1985; Godin et al., 2006; Fossen, 2010; Ganade de Araujo et al., 2013; proach during this era was that of simple shear with or without addi- Cottle et al., 2015). They also represent rheological and mechanical tional shortening/dilation across the shear zone. Pure shear was then anomalies that may be reactivated or otherwise influence the structural combined with simple shear to create more general subsimple shear evolution during later stages or phases of deformation, for example zones, first in the pioneering work by Ramberg (1975a,b) and later by ⁎ Corresponding author at: Department of Earth Science/Museum of Natural History, University of Bergen, Allégaten 41, N-5007 Bergen, Norway. E-mail address: [email protected] (H. Fossen). http://dx.doi.org/10.1016/j.earscirev.2017.05.002 Received 19 January 2017; Received in revised form 6 May 2017; Accepted 6 May 2017 Available online 08 May 2017 0012-8252/ © 2017 Elsevier B.V. All rights reserved. H. Fossen, G.C.G. Cavalcante Earth-Science Reviews 171 (2017) 434–455 Fig. 1. Two shear zones at micro- and mapscale. a) Shear band in highly porous sandstone (thin section of core from the Njord Field, offshore central Norway). Deformation oc- curred at shallow (few hundred meters) depth under un- consolidated conditions. b) Great Slave Lake shear zone (Canada), showing deflection of the Thelon-Taltson Magmatic Zone (TTMZ). Based on USGS map (2005). Passchier (1986), Tikoff and Fossen (1993), and Simpson and De Paor fabric (Figs. 1 and 2a). The geometry, orientation and relative move- (1993), and then to combine pure and simple shear in a three-dimen- ment of the walls are the boundary conditions that control the de- sional way, particularly in the framework of transpression and trans- formation within the zone. However, several processes may change the tension (Sanderson and Marchini, 1984). In this work we will review boundary conditions over time, for example changes in compaction the most useful and fundamental aspects of shear zones, their evolution caused by pressure solution and related loss of material in the zone from incipient to large structures, and discuss challenges that need to be (thinning), strain localization where margins are left inactive (thin- studied in the future. ning), inclusion of larger or smaller portions of wall rocks (widening), and interaction between adjacent shear zones (widening by linkage). 2. Definition and classification Hence, terms such as widening, constant thickness and thinning shear zones are commonly used. A shear zone is a zone in which strain is clearly higher than in the A significant distinction can be made between plane strain zones and wall rock, and whose margins are defined by a change in strain, typi- non-plane strain zones, i.e. zones involving two- and three-dimensional cally seen by rotation of preexisting markers or formation of a new strain, respectively. Plane strain implies no change in length along the Fig. 2. a) Schematic illustration of a simple shear zone, showing strain ellipses, new-formed foliation and two marker layers assumed to behave passively. b) Shear strain profile through the zone. c) Graph showing relationships between shear strain (horizontal axis) and the orientation of the strain ellipse and the two markers shown in (a) as well as strain (R). d) Natural shear zone (based on photo by Giorgio Pennacchioni) from deformed granitoid rock in the Tauern Window, Italian Alps, and e) a shear strain map with three shear strain profiles estimated from foliation orientation, assuming simple shear. Displacement d is given in terms of the local shear zone thickness. 435 H. Fossen, G.C.G. Cavalcante Earth-Science Reviews 171 (2017) 434–455 intermediate (Y) principal strain axis, and thus many aspects of plane Correspondingly, a brittle shear zone shows discontinuous deformation strain can conveniently be dealt with by considering the plane con- where originally continuous markers are broken up by slip surfaces taining the maximum and minimum principal strain axes (X and Z) (shear fractures) that cause discontinuities in the displacement field (only the rotation of preexisting line and plane markers requires 3D (Fig. 4b). A completely brittle shear zone would have undeformed considerations in this case). Plane strain, whether simple, sub-simple or portions of rock between these slip surfaces. Ductile-brittle shear zones pure shear, plots along the diagonal of the Flinn diagram (Flinn, 1962), contain both continuous and discontinuous deformation. In this sense, while 3D deformations produce off-diagonal constrictional or flattening drag associated with faulting creates a ductile-brittle shear zone, even if strains. However, if volume change occurs by compaction across the the deformation mechanisms involved are purely frictional (e.g., shear zone in combination with a plane strain deformation (such as Homberg et al., 2017). simple shear), the resulting plane strain will plot in the flattening field (Ramsay and Woods, 1973). 3. From simple shear zones to zones of 3D strain Shear zones can further be classified according to their dominant micro-scale deformation mechanism, where plastic (or crystal-plastic) 3.1. Simple shear and the ideal (Ramsay-type) shear zone shear zones, also referred to