K–Ar age constraints on the evolution of polydeformed fold–thrust belts: The case of the Northern Appalachians (southern Quebec)

Abstract

Precise geometric and temporal constraints are essential for understanding the evolution of fold–thrust belts. The lack of isotopic dating for shallow faults in the Northern Appalachian Taconic allochthons limits the interpretation of their structural evolution. New and more precise kinematic constraints have been obtained using a structural study that combined field work with the mineralogical, morphological and K–Ar isotopic analysis of clay-rich fractions (<0.2 and 1–2 μm) from fault rocks and host lithologies in selected fault zones within the southern Quebec allochthons. The validated K–Ar data define four age groups at ca. 490, 465–450, 410 and 360 Ma, which are, respectively, referred to as D1 and D2 Taconic deformation pulses, D3 Late Silurian–Early Devonian deformation, and D4–5 Acadian deformation. The recognition of discrete contractional pulses during the Taconic Orogeny suggests that folding and thrusting were initiated during Early Ordovician time (ca. 490 Ma) and culminated with the out-of-sequence imbrication of thrust stacks during Middle (ca. 465 Ma) to Late Ordovician time (ca. 450 Ma). The Silurian–Early Devonian subsidence (ca. 410 Ma) of the Appalachian foreland along wrench and normal fault systems occurred simultaneously with D3 hinterland-directed tectonic transport at the boundary between the external and internal Humber zones. Late Devonian Acadian imbrications (D4) in the Taconic allochthons of the Quebec Appalachians are reported here for the first time. The combination of a D4 contractional pulse and Silurian–Early Devonian D3 back-thrust faulting caused the exhumation of low-grade metamorphic rocks affected by an Early Devonian thermal event. Across the Laurentian margin, the Acadian contraction evolved from Middle Devonian deformation in the internal metamorphic domain to Late Devonian deformation in the allochthons. The relaxation of Acadian tectonic stress is characterized by D5 normal faults and related hydrothermal activity. The recognition of an Acadian contraction event in the southern Quebec Appalachians challenges the traditional view of a gradual NW-directed progression of faulting in the Taconic thrust belt, and allows for a new reconstruction that bridges the Appalachians of mainland Canada and Newfoundland.