Читать книгу Continental Rifted Margins 1 - Gwenn Peron-Pinvidic - Страница 35

Folds can occur at various geologic scales and in various tectonic settings. Intuitively, folds are often associated with shortening rather than extension. However, they may also develop in extensional settings in specific structural contexts, such as in fault propagation folds (folding above a fault tip) or related to the intrusion of igneous material (e.g. above a laccolith). They can also be associated with differential compaction of sedimentary layers. Constrictional strain can also produce folds and compressional geometries in basins under regional extension (e.g. SW Norway Devonian basins, Osmundsen Andersen 2001). Folds are analyzed based on their overall geometry. The three main categories of folds are monocline, anticline and syncline. Geometrically, they are described by how the limbs dip to the hinge axis (Figure 1.24).

Figure 1.23. a) Illustration of a schematic boudinage structure; b) field examples of “pinch-and-swell” boudins from Norway with amphibolite boudins within quartz schists (top: Finnmark, Norway) and a deformed pegmatite dike (bottom: Roan, Norway) (photo credits © Steffen Bergh and © Haakon Fossen) and c) an offshore seismic reflection example from the Gabon rifted margin (source: modified from Clerc et al. 2018)

Figure 1.24. a) Illustration of the main fold geometries; b) field example photos from Portugal (photo credits © Haakon Fossen) and c) seismic refection profile examples of fault-propagation folds associated with different sets of “simple” and “conjugate” normal faults, from the Inner Moray Firth, UK (source: from Lapadat et al. 2017)

Further reading.– The above descriptions are abbreviated and often simplified. If interested in reading and learning further, the reader is referred to the following list of publications and references:

 – General: (Wernicke 1985; Lister et al. 1986; Buck 1988, 1991; Lavier et al. 1999; Axen 2004; Davis and Kusznir 2004; Reston 2007, 2009; Reston and McDermott 2011; Whitney et al. 2014; Jackson et al. 2017; Brun et al. 2018; Rotevatn et al. 2019).

 – For structural geology definitions, we recommend the excellent book (Fossen 2010).