Continental Rifted Margins 1. Gwenn Peron-Pinvidic
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Название: Continental Rifted Margins 1

Автор: Gwenn Peron-Pinvidic

Издательство: John Wiley & Sons Limited

Жанр: Физика

Серия:

isbn: 9781119986911

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СКАЧАТЬ 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.

Schematic illustration of a detachment fault accommodating displacement at low angles. Photograph of a field example of the Nordfjord-Sogn detachment zone.

      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)

Schematic illustration of extract from a seismic reflection profile in the mid-Norwegian rifted margin. Schematic illustration of a shear zone.

      1.3.2.4. Metamorphic core complexes

Schematic illustration of a metamorphic core complex.

       1.3.2.5. Boudinage СКАЧАТЬ