Structural evolution of the Cape Smith Thrust Belt and the role of out‐of‐sequence faulting in the thickening of mountain belts

Abstract

Out‐of‐sequence thrusting represents a mechanism capable of heterogeneous internal deformation in thrust belts and was largely responsible for internal thickening in the early Proterozoic Cape Smith Belt. The south verging thrust belt is marked by a complex history of accretion, internal deformation, thickening‐induced metamorphism, uplift, and erosion. An overall chronology for thrust belt evolution is built around the timing of thermal peak metamorphism. Prethermal peak deformation includes (1) initial piggyback stacking of basalt‐dominated thrust sheets above a basal décollement localized at the basement‐cover contact; (2) greenschist to amphibolite grade metamorphism accompanied by development of a ductile shear zone at the base of the thrust belt; and (3) out‐of‐sequence thrusting, precipitating the metamorphic thermal peak both above and below these faults. Synthermal peak out‐of‐sequence thrusting was responsible for accretion of autochthonous crystalline basement slices and eventual abandonment of the external basal décollement. Postthermal peak deformation is characterized by the continued development of out‐of‐sequence thrusts which appear to young toward the hinterland (north). Internal thickening by development of flattening cleavages or folding appears to be regionally insignificant when compared to that accomplished by the out‐of‐sequence faults. This suggests that (1) an eroding and accreting thrust wedge may be able to maintain its critical shape by out‐of‐sequence thrusting; and (2) this style of internal deformation is favored in belts dominated by relatively strong material (e.g., basalt, as in the Cape Smith Belt).