Suprastructure/infrastructure transition, east-central Cariboo Mountains, British Columbia: geometry, kinematics and tectonic implications

Abstract

In the Cariboo Mountains of east-central British Columbia, syn-metamorphic D 2 folds of the Proterozoic Kaza Group change from upright, open and symmetrical to gently inclined, tight (locally isoclinal) and SW-verging. Plunge measurements and change in metamorphic grade show that the transition occurs with paleo-depth in the orogenic belt. This ‘suprastructure-infrastructure’ transition is attributed to a downwardly increasing component of non-coaxial strain accumulated during northeastward underthrusting of allochthonous marginal basin sediments and autochthonous distal North American continental margin sediments beneath more proximal sediments of the continental margin. A comparison of the structural thickness of the Kaza Group with its original thickness reveals that a greater amount of structural thickening is represented at deeper structural levels than at immediately overlying shallower levels. This geometry requires that shallow structural levels to the west be separated from deeper structural levels by thrust faults. A belt of pre-metamorphic-peak, SW-verging thrust faults in the Quesnel Highlands may be the shallow level manifestation of infrastructure thickening. Approximately 60 km of NE displacement of rocks at deep structural levels with respect to a fixed suprastructure can account for both the infrastructural thickening in the cast-central Cariboo Mountains and the geometry of thrust faults in the Quesnel Highlands.