**3. Architecture and embryonic oceans and hyperextend rift basins**

Although the Wilson cycle is often used to describe the continental collision zones as formed by the closure of broad oceans floored with Penrose-type crust [42], there are numerous orogenic belts (e.g., Pyrenees, Alps, and European Variscides) having conjugate precollisional margins separated by immature extensional zones (i.e., narrow oceans or hyperextended continental rift basins) [7, 29, 34, 43, 44]. Immature extension systems are rift systems that begin when the continental crust has been stretched to complete embrittlement, typically at a crustal stretching factor of about 3–4 [45], and whose development stopped before or with the initiation of seafloor spreading [43, 44, 46, 47] (**Figure 1**). These rift systems involve thinning continental crust to the exhumation of the subcontinental lithospheric mantle at the floor of rift basins [43, 44, 46, 47]. In these basins, the entire remaining crust and the uppermost part of the subcontinental mantle are subject to intense hydrothermal circulation that forms sericite and illite in the crustal rocks [48–50] and serpentine, chlorite, and talc up to 4–6 km deep in the lithospheric mantle [51–54]. The hyperextension of the continents can also lead to decompression melting of the underlying asthenospheric mantle [55], commonly without the formation of mid-ocean ridge basalt (MORB)-type melts, or they are only partially extracted and tend to stagnate in and

#### **Figure 1.**

*(a) Various stages of extension and corresponding physical properties of the lithospheric mantle at the center of the rift. (b) Architecture and lithology of a typical magma-poor rifted margin [44].*

fertilize the overlying lithospheric mantle [56]. This process tends to homogenize (on a large scale) the uppermost lithospheric mantle (~30 km) beneath the rift basins into plagioclase-bearing lherzolite [41, 56]. However, growing evidence suggests that fertilization of the lithospheric mantle beneath the rift basins is uneven and gradually increases from the proximal to the unstretched continental part to an ideal homogeneous plagioclase-bearing peridotite beneath the distal part of the rift basin [41].

The fertilized mantle under immature oceanic basins, such as beneath the southern part of the Porcupine Basin [57] or at hyperextended rifted margins such as the Iberian margin [58], the Newfoundland margin [59], and the Flemish Cap [60], is also characterized by a reduction in seismic velocities of about 1% at pressures up to 1 GPa and by >2% at higher pressures [44]. Although serpentinization is likely the main cause of the velocity decreases up to 6 km below the seafloor, it may not be responsible for reducing the seismic velocity at greater depths as the serpentine becomes unstable [44]. However, the presence of plagioclase in the fertilized mantle strongly affects the rheology of the lithospheric mantle because plagioclase is weaker than pyroxene and olivine [44]. As a result, the fertilized mantle behaves semi-brittle (i.e., the plagioclase stability field) between 18 and 40 km below the embryonic oceans and hyperextended rift basin. This mantle domain, where deformation can be accommodated along localized anastomosing shear zones, is broader than in the lithospheric mantle beneath the bounded continental margins and could have significance as a stress guide for subduction initiation of rift basins beneath passive continental margins at greater depths up to 30 km [44].

The absence of significant subduction-related magmatism in orogens formed from Ampferer-type subduction zones is interpreted because of the narrow width of the closing ocean, which did not allow significant decompression and/or flux melting [61, 62]. Therefore, the mantle beneath orogens resulting from the closure

#### **Figure 2.**

*Two end-members proposed for the Wilson cycle in the North Atlantic as it is resulting from the closure of both broad, mature oceans (the Scandinavian Caledonites), and narrow oceans (<500 km) or immature, hyperextended rift systems (the Variscides of Western Europe) [44].*

of a hyperextended rift basin or narrow embryonic ocean is likely fertile because it is hydrated and enriched in mobile constituents derived from subducting sediments, oceanic crust, and dehydrating serpentinite but lacks significant flux melting [63]. This fertile mantle could provide fusible components for intense magmatism during subsequent collapse (magma-rich orogenic collapse [43, 44]. This is shown in the widespread mafic to acidic intrusions, for instance in the crust in the Variscan region, Basin and Range province, and Canadian Cordillera [64–67]. Conversely, orogenic belts formed by the closure of a large ocean are associated with intense flux melting within the mantle wedge due to the release of substantial fluids from dehydration of the large, subducted slab and decompression melting of the hot asthenosphere that rises to compensate for the down dragged mantle wedge material by the slab [68–70]. These melting processes form island arcs at the sea-flower or volcanic arcs at the continental margin [71] and deplete the source mantle wedge

[72]. Orogens formed by the closure of a large ocean may therefore be underlain by a relatively depleted mantle [43]. Therefore, the subsequent collapse of these orogenic belts is devoid of magma (magma-poor collapse). The Western Europe-North Atlantic region is an ideal example to examine how shortening or incomplete Wilson cycles may differ from classic Wilson cycles, as it consists of orogens resulting from the closure of both narrow oceans (< 500 km) or immature, hyperextended rift systems (the Variscides of Western Europe) and broad, mature oceans (the Scandinavian Caledonides) (**Figure 2**) [44, 73].
