Gwenn Peron-Pinvidic

Continental Rifted Margins 2


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temporal distribution of fault activity during the rifting. Distinctive modes of fault development and emplacement have resulted into three main models proposed to explain the migration of the extension suggested by the available data. In the following section, we introduce these models designed from observations at the West Iberian Margin, and likely explain how magma-poor margins might have formed.

      Model M1, crustal depth-dependent thinning. Driscoll and Karner (1998) and Davis and Kusznir (2004, pp. 92–136) suggested that both the amount and the mechanisms of extension vary with depth across the crust, due to the different rheologies of the upper and the lower crust. In this way, the extension occurs by faulting in the brittle upper crust, whereas the lower crust deforms through ductile processes, not just stretching but also displacing or flowing, thus thinning the crust much more than indicated by the faulting (Figure 1.5). In some cases, the interpretation of eroded fault block crests and the dominance of shallow water sediments has led to the idea that much of the DDS occurred from the displacement of the lower crust late during the rifting process (Clift and Lin 2001, pp. 489–510; Davis and Kusznir 2004, pp. 92–136). In this interpretation, all the brittle deformation is imaged and occurred during a single phase, so the synrift sediments have the same age across the margin (Figure 1.7).

      Model M2, cross-cutting polyphase faulting. An alternative perspective is that the crust thins by the same amount at all levels, but that not all the upper crustal extension is correctly identified. In this polyphase faulting model, extension might be underestimated where young faults cut and displace old faults (Reston 2005; Reston and McDermott 2014) as extension occurs through distinctive phases of deformations that focus with time into the rift axis. This model results in complex structural and stratigraphic geometries which means most of the earlier faults cannot be identified from seismic reflection profiles, thus explaining the extension discrepancy (Reston 2009; Figure 1.5). Seismic modeling of the resulting complex geometries confirms that earlier faulting is indeed likely to be misinterpreted, for example as eroded fault block crests (McDermott and Reston 2015). As faulting may have started at the same time across much of the margin, but focused during rifting towards the rift axis, the earliest synrift should be present across the margin, but the youngest synrift should be present only near the axis of breakup, being of the same age as local post-faulting sediment on the flanks (McDermott and Reston 2015). The initial top basement and pre-rift are dismembered and only preserved locally, as much of the top basement surface comprises early fault surfaces (Figure 1.7).

      The sequential faulting, as an inherently asymmetric process, might also explain the development of asymmetry between conjugate margin pairs. As proposed by Ranero and Pérez-Gussinyé (2010), sequential faulting might develop during early rifting when the continental crust is still >20 km thick. However, this early development conflicts with the apparent link between asymmetry development and crustal embrittlement, which requires asymmetry to develop only once the crust is thinner than 10 km (Reston and Pérez-Gussinyé 2007; Reston 2009). However, dynamic models propose a later onset of the development of asymmetry (Brune et al. 2014), when the crust becomes thinner than 20 km. The latter scenario is consistent with 3D seismic-derived interpretations showing that extension migrated on a 3D multi-fault variant of the rolling hinge model, only once slip at low-angle was allowed (Lymer et al. 2019). Brune et al. (2014) proposed hot ductile lower crust as a rooting zone, while Lymer et al. (2019) suggested that the recognized serpentinized mantle beneath S (Bayrakci et al. 2016) might represent the weak root zone.

      The fundamental disparities between the three models – in terms of timing of faulting, number of faulting phases and rheologies – demonstrate that despite decades of exploration of the WIM, our knowledge of rifting and breakup remains fundamentally incomplete, as long as detailed timing of the geological events at rifted margins remains undefined.

      The diversity of existing current models developed at the WIM shows that we still fundamentally do not know how the continental crust thins to zero, exposing the Earth’s mantle to the surface during rifting, leading to eventual continental breakup. The models presented in the previous section (Figure 1.7) are all potentially viable but imply fundamental differences in the mechanisms of thinning of the crust. The differences between these models can be summarized in three major, yet unanswered, questions, emerging from our current knowledge of the structures of the Galicia Margin, and more globally related to the development and evolution of magma-poor rifted margins.

Schematic illustration of the models proposed to explain fault development and the evolution of the west Iberian margin.