Gwenn Peron-Pinvidic

Continental Rifted Margins 1


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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).

Schematic illustration of boudinage structure. Schematic illustration of the main fold geometries.

      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).

       1.3.3. Main basin types

       1.3.3.1. Grabens, half-grabens and horsts

Schematic illustration of the structural geometries of a graben, horst and half-graben.

      1.3.3.2. Pull-apart basins

Schematic illustration of the main structural geometries associated with a pull-apart basin.

       1.3.3.3. Sag basins

      In geology, the term “sag basin” is used to describe deep, large-scale basins with an overall circular/elliptical shape (Kingston et al. 1983; Middleton 1989). Interior cratonic basins are often referred to as sag-type basins, displaying accumulations of sediments in an overall oval, long-lasting depression. The formation of sag basins is hypothesized to be driven by long-lasting thermal subsidence (Sleep 1971) and not involve major faulting. In the recent literature, the term “sag basin” has been widely used to describe basins encountered in distal rifted margins, usually referring to large-scale (>100 km wide) basins displaying conformable sedimentary sequences that are draped over underlying faulted topography (see the expanded description in Chapter 2).

       1.3.3.4. Linkage of rift faults and basins