Deformation-induced inverted metamorphic field gradients: an example from the southeastern Canadian Cordillera

Abstract

Exhumed middle crustal rocks of the hinterland of the southeastern Canadian Cordillera were deformed and metamorphosed during Mesozoic and Tertiary progressive crustal thickening. High structural levels were transported to the northeast relative to lower levels through a combination of thrusting and ductile non-coaxial flow. Progressive growth of the orogen and advance of hinterland rocks toward the foreland are revealed through analysis of diachronous metamorphism and associated deformation. At the deepest exposed level, allochthonous rocks of the orogen (Selkirk allochthon) structurally overlie Early Proterozoic basement and younger cover rocks (Monashee complex) that are correlated with autochthonous North American crust underlying the eastern Foreland Belt. The crustal zone, which marks the boundary between the Selkirk allochthon and Monashee complex, exhibits an inverted metamorphic field gradient. New data presented in this paper refute previous interpretations, which asserted that the metamorphic inversion is a result of the downward transfer of heat from the allochthon to the underlying Monashee complex. Rather, the inversion is attributed to synmetamorphic non-coaxial progressive deformation. A model is proposed in which substantial northeastward directed shear strain and attendant attenuation led to lateral transfer of rocks thereby preserving evidence of strongly diachronous deformation and an apparent inverted metamorphism.