Abstract

The Cap-Chat mélange is a regional-scale dismembered unit that occurs close to the Quebec Appalachian front. The mélange records early (D1, Taconian) and late (D3, Acadian) regional deformations. The greater degree of disruption of the mélange compared with surrounding units is due to the superimposition of D2 structures that are rare outside the mélange. D2 structures include both layer-extensional features such as block disruption along shear zones and faults and layer-contractional features such as folds that record the transtensional collapse of the Taconian orogenic wedge along a rheologically weak deformation zone. The Cap-Chat mélange is interpreted as the westernmost remnant of a zone of mechanical decoupling in the upper crust, which allows normal or transtensional faulting in the inner parts of the orogen. This paper presents evidence that beyond the general complexity of mélange units, geological observations may still contribute to unraveling their tectonic history and that transtensional reactivation of original thrust faults is a viable mechanism in the frontal parts of orogens.

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