Continental Rifted Margins 1. Gwenn Peron-Pinvidic
Читать онлайн книгу.
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 structures
In structural geology, boudinage refers to a deformed structure produced during the extension of a rock that is characterized by the presence of sheared rigid parts enclosed in a less competent surrounding matrix (Figure 1.23). The “boudins” correspond to pieces of the more rigid rocks that were once part of a continuous layer and have been broken into individual units, isolated laterally by anastomosing shear zones deformed in the ductile or semi-ductile stress-field. Some boudins exhibit a “pinch-and-swell” geometry. Boudinage structures can occur at all scales from microscopic to lithospheric. In rifted margin studies, it has been proposed that the continental lithosphere may deform at the regional scale under such a deformation mode. It is usually associated with specific geological settings including high thermal conditions.
Figure 1.22. Schematic illustration of the evolution of low-angle normal faults (LANFs) with the domino-block rotation model and the rolling-hinge model (source: modified from Ricketts et al. 2015)
1.3.2.6. Folds