The Selkirk fan structure of the southeastern Canadian Cordillera

Abstract

The Selkirk fan structure evolved by superposition of two distinct phases of deformation upon strata previously involved in nappe formation (phase I). Phase II folds are dominant on the southwestern flank of the Selkirks where the folds are strongly overturned toward the southwest. Phase III folds are dominant on the northeastern flank where the strata are overturned toward the northeast. The fan structure is located where northeastward-dipping phase II axial surfaces are overprinted and transposed by steeply dipping to vertical phase III axial surfaces. Granodioritic plutonism and the main growth of metamorphic porphyroblasts occurred after phase II and before the onset of phase III. Depths of burial compatible with the peak of regional metamorphism were maintained at least until the later stages of phase III deformation. The evolution of the fan structure may be explained in terms of initial underthrusting of the southwestern flank of the Selkirk terrane by the Shuswap metamorphic complex followed by underthrusting of the northeastern flank by basement rocks of the Rocky Mountain foreland.