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In the Monts d'Arree (western Brittany, France) a high-strain slate belt is well-exposed. The slate belt is located in the Central Armorica Terrane, a low-grade middle- to upper-crustal domain in the Armorican Massif, composed of a Cadomian basement and its Neoproterozoic and Palaeozoic metasedimentary cover sequence. The slate belt consists of highly deformed siliciclastic rocks of the Pridolian to Lochkovian Plougastel Formation. A structural analysis has demonstrated that the slate belt primarily reflects a coaxial, contraction-dominated deformation history, resulting from a top-to-the-NW shearing on top of a weakly dipping decollement. It resulted in NW-verging folding and a pervasive cleavage development, giving rise to a pronounced subvertical mechanical anisotropy. Only during the later stages of deformation history, incipient strain partitioning on this anisotropy lead to the development of punctuated strain heterogeneities, consistently reflecting dextral, belt-parallel, strike-slip strain. The deformation largely occurred prior to the emplacement of the early Carboniferous granitoid intrusions in Central Armorica, and can therefore be correlated with the late Devonian–early Carboniferous ‘Bretonian’ tectonometamorphic event. The kinematics, inferred in the Monts d'Arree, moreover, are consistent with the top-to-the-NW thrusting and nappe stacking inferred in the Leon Terrane, situated to the northwest of the Central Armorican Terrane. These new insights allow linkage of the early Variscan, contraction-dominated deformation in Central Armorica to the closure of the Rheic Ocean and the continental collision of the Leon microcontinental block with Armorica. Our study also reveals the importance of the ‘Bretonian’ tectonometamorphic event in the entire north-western parts of Central Armorica; the main – cleavage-forming – deformation was diachronous and followed by subordinate wrench-related deformation. |
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