Structural evolution of the southern Swayze greenstone belt, Superior Craton: Implications for the Neoarchean crustal dynamics

Abstract

Numerous studies suggest that the Neoarchean may have been a transitional period from vertical tectonism to horizontal tectonism in crustal dynamics. The former is characterized by spontaneous gravitational redistribution of crustal materials, while the latter is characterized by partitioning of far-field tectonic stress into localized high shear zones and regional-scale strike-slip motion. However, how these combined crustal dynamics are manifested in the geological record remains poorly understood. In this study, a deformation analysis was carried out in the southern part of the Neoarchean Swayze greenstone belt, southeastern Superior Craton, as well as in the enclosing Ridout deformation zone. Our structural analysis reveals four generations (G₁  – G₄) of ductile deformation. G₁ deformation is preserved only in the pre-tectonic granitoid intrusion. G₂ deformation established the dome-and-keel architecture of the Swayze greenstone belt and is associated with regional F₂ folding, the S₂ foliation subparallel to the granitoid–greenstone boundary and the L₂ stretching lineation which exhibits a reversal in the plunge direction on the opposing granitoid–greenstone boundaries. The G₂ deformation zones show a granitoid-up/greenstone-down sense of shear. G₃ deformation is associated with oblique sinistral movement on the Ridout deformation zone possibly due to regional sinistral transpression. G₄ deformation is a reactivation event of pre-existing weak planes in the form of transcurrent dextral shearing. The kinematics of G₂ structures is interpreted to be the result of combined sagduction/diapirism and regional dextral shearing, thus implying a crustal condition that is “weak” enough to allow spontaneous crustal-scale gravitational readjustments while at the same time “strong” enough to transfer regional tectonic stress. In this sense, both styles of tectonism may have operated simultaneously in the Swayze greenstone belt. Our results support the hypothesis that the Neoarchean era is a transitional period during which the crustal dynamics evolved from being mainly vertical to horizontal and that the Swayze greenstone belt preserves evidence for the co-operation of both processes. Furthermore, our analysis, including temporal constraints, recognizes a comparable structural history between the Ridout deformation zone in the Swayze greenstone belt and the Cadillac-Larder Lake deformation zone in the Abitibi greenstone belt. However, discrepancies exist in terms of the vertical kinematics and the position of the deformation zone with respect to the syntectonic Timiskaming-type sedimentary assemblage. Therefore, we propose that the two deformation zones may have been initiated as separate entities rather than a coherent crustal-scale deformation corridor.