at its decollement level (Figure 1.20). The geometry, either concave-upward or concave-downward, typically includes multiple segments – the most active being the ones at higher angles, whereas the lower-angle segments are instead interpreted as segments that have been flexurally rotated during unroofing after their main phase of activity. Whether or not the low-angle segments can actually be considered as active remains debated, as these are not explained by Andersonian fault mechanics (see the section above) (Axen 2004). Detachment faults are often understood to begin at middle-lower crustal depths in zones with weak rheologies, where mylonitic structures form. Ductile shear is the main deformation mechanism at depths greater than 20 km, whereas brittle behavior dominates at shallower depths. Detachment faults are often associated with the genesis of metamorphic core complexes in continental settings and oceanic core complexes in oceanic domains (see below). Key onshore examples include the Nordfjord-Sogn Detachment in south-west Norway, the Snake Range detachment system of the Basin and Range Province in the western USA and the Whipple Mountains detachment in California.
1.3.2.3. Shear zones
Shear zones may develop in rocks submitted to intense deformation, where some planar to sub-planar zones are characterized by high strain rates compared to the surrounding rocks that have undergone lower finite strain (Figure 1.20). Shear zones are often considered to be the deeper crustal counterpart of the shallow brittle faults. The motion of the more rigid surrounding blocks may imply a rotational, non-coaxial component in the shear zone. Depending on the rheological context, shear zones can be brittle, semi-brittle, ductile or a combination. Ideally, the deformation is concentrated either in a narrow fracture (brittle) or distributed over a wider zone (ductile). The change from brittle to ductile behavior is classically associated with depth, with the development of intermediate deformation styles where brittle fracturing coexists with plastic flow. However, other parameters can strongly influence that transition, such as the local lithospheric thermal state, the presence of fluids, compositional changes (e.g. serpentinization), strain rates, stress field orientation and compositional anisotropies.
Figure 1.19a. Diagram illustrating a detachment fault accommodating displacement at low angles, with slightly to highly deformed rocks in its footwall (i.e. mylonitic rocks, breccias, cataclasites), and high-angle normal faults bounding tilted blocks on its hanging wall
Figure 1.19b. Photo showing a field example of the Nordfjord-Sogn detachment zone (NSDZ), which is a major detachment fault in Norway. WGR: Western Gneiss region (source: photo by Per Terje Osmundsen)
Figure 1.19c. Extract from a seismic reflection profile in the mid-Norwegian rifted margin (top left) with the interpreted structures (lower left) including a major regional detachment fault and tilted fault blocks. Locations on the map inset, upper left (source: from Peron-Pinvidic and Osmundsen 2020)
Figure 1.20. Illustration of a shear zone: a) diagram illustrating the concentration of the deformation in a preferred zone that generates the formation of highly deformed rocks; b) field photo (photo credits © Haakon Fossen) of the Nordfjord-Sogn detachment (Rutledal, Norway) and c) c1: schematic cross-section illustrating the structural setting of the south-west Norway Devonian basins (source: Osmundsen and Andersen 2001); c2: extract from a seismic reflection profile in the Stord Basin in the North Sea showing the possible presence of shear zones at basement depths (source: Peron-Pinvidic and Osmundsen 2020). Locations on the map inset, lower right
1.3.2.4. Metamorphic core complexes
When a region is submitted to large amounts of extension, mid- to lower-crustal levels can be dragged out from beneath the brittle fractured upper crust. The resulting dome-shaped structure is called a metamorphic core complex (Whitney et al. 2014; Brun et al. 2018). Typically, the exhumation process includes a concave-downward detachment fault, exhibiting ductile deformation in the footwall with amphibolite- to greenschist-facies metamorphism, and brittle deformation in the hanging wall with high-angle normal faults (HANFs) and tilted unmetamorphosed crustal blocks. These HANFs often merge with the detachment fault at depth (Figure 1.21). Depending on the lithospheric parameters (e.g. thermal state, extensional rate), the core complex may be capped by a shear zone rather than a simple fault plane, with brittle to ductile deformation features. Deformation products associated with such structures are multiple and can include breccias, mylonites and phyllonites. Metamorphic core complexes are encountered in various extensional settings and are often interpreted in seismic data of rifted continental margins.
Figure 1.21. Illustration of a metamorphic core complex
CONTINUATION OF CAPTION FOR FIGURE 1.21.– a) Schematic representation of a metamorphic core complex with a window of exhumed basement capped by a detachment fault and overriding tilted blocks flanked by high-angle normal faults (HANF) (source: modified from Osmundsen et al. 2003 and Fossen 2010); b) field example with map, sections and photos from the Western Gneiss Region in southwestern Norway (A, B) and focus on the Gulen Metamorphic core complex (MCC) with outcrop photos (right-hand inset photos). NSDZ: Nordfjord-Sogn detachment zone. BASZ: Bergen Arc Shear Zone (source: from Wiest et al. 2019) and c) seismic reflection profile (TGS) from the Trøndelag Platform; in time (s-twtt), without (top) and with interpretation (bottom) (location on the map inset on the right) (source: from Peron-Pinvidic and Osmundsen 2020).
The rolling-hinge model for metamorphic core complexes (Buck 1988; Wernicke and Axen 1988; Lavier et al. 1999) includes the definition of a large-scale concave-downward detachment fault which evolves from high to low angles as the footwall flexes upward during unroofing from below the hanging wall (Figure 1.22). This model is often used to explain basement exhumation in continental rifted margin and oceanic ridge studies. Reston and McDermott (2011) further refined the rolling-hinge concept by having successive detachment faults at the rift switch polarity and thus cut into their predecessor’s footwall (the so-called “flipping” or “flip-flop” model). The resulting alternating dip orientation of the main detachment then provides an elegant model to explain how extensive surfaces of symmetric exhumed basement can be generated by an asymmetric tectonic structure (for further information, see Chapter 2).
1.3.2.5. Boudinage
