Continental Rifted Margins 1. Gwenn Peron-Pinvidic
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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).
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).
1.3.3. Main basin types
Rifts and rifted margins are usually composed of several basins of various types and linked in various ways. Below is a list of the major types of basins that can be encountered in rift and rifted margin studies. Again, this list does not aim at being entirely complete, but representative of the main geometries encountered in rifts and rifted margins.
1.3.3.1. Grabens, half-grabens and horsts
Grabens and half-grabens are the most common type of sedimentary basins generated in extensional settings. A graben is typically identified as a valley flanked by distinct topographic escarpments. Structurally, grabens are formed by arrays of oppositely dipping normal faults. In the ideal case, the flanking faults are parallel and antithetic and grow at the same rate so that there is no rotation of the hanging wall. The overall downward movement of the common hanging wall leads to the formation of a symmetric depression: the graben (Figure 1.25). However, faults rarely grow at the same rate, so grabens are usually asymmetric. If several faults form, the hanging wall platform can be further dissected and can generate a horst (Figure 1.25). Half-grabens correspond to the topographic depression developed on the tilted hanging walls of normal faults.
Figure 1.25. Diagram illustrating the structural geometries of a graben, horst and half-graben
1.3.3.2. Pull-apart basins
In some cases, extension can occur related to strike-slip faults. In regions with multiple segments of strike-slip faults, the lateral displacement between the faults will lead to stepovers and, eventually, the development of pull-apart basins – also named strike-slip basins (Figure 1.26). Typically, pull-apart basins are strongly compartmentalized, with each individual basin sharing similarities with discrete intracontinental rifts. The standard basin geometry corresponds to a rhomb-shaped graben or half-graben. In theory, these basins tend to be fairly deep with a fast subsidence. Type examples of strike-slip faults with pull-apart basins include the San Andreas Fault, the Dead Sea Transform and the North Anatolian Fault (Şengör et al. 2005). The most cited examples of strike-slip rifts include the Salton Sea, the Death Valley, the Jordan Valley-Dead Sea Rift – Gulf of Eilat and the onshore northernmost part of the Red Sea Rift.
Figure 1.26. Diagram illustrating the main structural geometries associated with a pull-apart basin, at its early stage of development (source: modified from Wu et al. 2009, based on analogue modeling results)
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
Faults rarely form as isolated features and a rift system is built through the activity of several families, or populations, of faults. With ongoing extension and crustal thinning, faults and basins may interact and link in various geometries. Depending on the scale of the study, various terms are used in the literature to refer to these distinct areas. The series of faults that interact and/or accommodate extension under similar structural settings are often known as “fault complexes”, “fault systems” or “fault zones”. At one stage, as faults grow during ongoing extension, they may come close to each other and specific interactions are then observed. Distinct vocabulary is then used to characterize the faults: unlinked faults that do not mechanically interact, soft-linked faults that form an overlapping structure but are not physically connected and hard-linked faults that are physically connected. The areas