Wrench‐Related Dome Formation and Subsequent Orogenic Syntax Bending in a Hot Orogen (Variscan Ibero‐Armorican Arc, the Ouessant Island, France)

Abstract

During the Carboniferous collision stage, the West European Variscan orogen was affected by oblique convergence, wrenching, plate-scale oroclinal bending, and widespread exhumation of the deep crust. One of these exhumed units forms the Léon dome located on the northern flank of the Ibero-Armorican Arc in the western part of the Armorican massif. Structural field data from the Ouessant Island reveal kinematic changes between 330 and 300 Ma that affected the northwestern margin of the Léon dome. This margin underwent two ductile deformation phases with opposite strike-slip shear senses. Dextral strike-slip wrenching combined with orogen-normal shortening, orogen-parallel stretching, and local top-to-the-NE shearing occurred during the Léon dome formation and exhumation of its gneissic core at 330-310 Ma. During tightening of the Ibero-Armorican Arc at~300 Ma, more localized sinistral wrenching reactivated the northwestern boundary of the Léon dome. The resulting N70°E trending sinistral Porspoder-Ouessant shear zone together with its conjugate dextral South Armorican shear zone controlled the lateral escape of the Léon-North-Central Armorican rigid tectonic wedge triggered by indentation of the Cantabrian orocline at the core of the Ibero-Armorican Arc.

Transpressive Deformation in the Southern European Variscan Belt: New Insights From the Aiguilles Rouges Massif (Western Alps)

“…Activation of the older branches of the EVSZ is coeval with the onset of sinistral transpressive deformation in the western sector of the Iberian Massif dated at~340 Ma (U-Th-Pb on monazite and zircon; Pereira et al, 2008Pereira et al, , 2010. Transpression in this sector of the Variscan Belt is interpreted to be a consequence of complex polyphasic indentation of a Gondwana promontory (Dias et al, 2016) and to orogenic syntaxial bending (Authemayou et al, 2019). The EVSZ may have contributed to this process during its early stages and subsequently facilitated formation of a second arc that characterizes the eastern sector of the Variscan Belt.…”

Strain Partitioning Within Bending Orogens, New Insights From the Variscan Belt (Chiroulet‐Lesponne Domes, Pyrenees)

“…The core of the arc (i.e., the Cantabrian Zone) is characterized by contractional structures, such as arc‐radial folds and steep E‐W striking thrusts (e.g., Bahamonde et al., 2007; Blanco‐Ferrera et al., 2017; García‐López et al., 2018; Merino‐Tomé et al., 2009; Pastor‐Galán et al., 2014; Pérez‐Estaún et al., 1988, Figure 1). To the contrary, the branches of the arc (e.g., Armorican Massif, Central Iberian Zone) exhibit arc‐parallel stretching, accommodated by both lateral wrenching and extension (e.g., Authemayou et al., 2019; Díez Fernández & Pereira, 2017; Gapais et al., 2015; Martínez Catalán et al., 2007). Based on this apparent opposition, it has been proposed that the cause of oroclinal bending is an active buckling at lithospheric scale that was responsible for contrasted mass transfers in the arc, depending on the relative position in the inner and outer arc (Gutiérrez‐Alonso et al., 2012).…”

Status of Basement Terranes in Orogens: Insights From the Cadomian Domain of the North Armorican Variscides, Western France

“…The southerly inclined metamorphic series on the cross‐section in Figure 2d recorded epizonal‐mesozonal P/T conditions that increased gradually northward from the Paleozoic series (pyrophyllite‐chlorite facies) toward the structurally lower part of the upper crustal pile (sillimanite/migmatite facies) without any gap or reverse position of metamorphic gradients (Jones, 1994; Paradis et al., 1983; Schulz, 2013). For clarity, the concept of two distinct juxtaposed metamorphic domains is still maintained in the present work, but in terms of infracrustal (LMD) and supracrustal (CAD‐Morlaix) domains forming a quasicontinuous and coherent crustal section, exhumed on the southern limb of the regional‐scale LMD dome (Authemayou et al., 2019). These two domains occur on both sides of an arbitrary map‐boundary in the Brioverian series (Figure 2d).…”

“…This tectonic contact is variously interpreted as either a dextral transcurrent shear zone (Balé & Brun, 1986) or a major thrust (Ballèvre et al., 2009; Faure et al., 2008, 2010; Rolet et al., 1986). However, the thrust contact that should have supported the allochthonous‐type models has never been so far documented in the field (Authemayou et al., 2019; Faure et al., 2010; Le Gall et al., 2014). Additional conflicting evidence against the allochthonous hypotheses come from the metamorphic pattern observed on the southern flank of the LMD dome, at the transition to the CAD (Figures 2a and 2d).…”