Structural history of continental margin sediments beneath the Bay of Islands Ophiolite, Newfoundland

Abstract

Deformed continental margin sediments of the Curling Group underlie the Bay of Islands Ophiolite in the Humber Arm allochthon of western Newfoundland. Within the allochthon, tectonic slices of sediments are separated by zones of mélange. The earliest structures in the slices are synsedimentary features produced by soft-sediment deformation on the continental slope or rise. Later, west-facing asymmetrical F1 folds without penetrative axial plane cleavage were probably produced during the emplacement of the allochthon in the Middle Ordovician Taconic Orogeny. Associated extensional structures include shear-fracture and extension-fracture boudins. Pervasive cataclastic shearing of shale and boudinage of competent beds produced mélange between slices. Shear surfaces and the original bed-parallel fissility of the shale both contribute to the anastomosing fabric of the mélange matrix.A subsequent folding event of probable Acadian age produced upright to moderately inclined F2 folds with axial-plane cleavage increasing in intensity eastwards across the allochthon. This event refolded the folds, thrust slices, and mélange zones produced during emplacement. Later, gentle cross folds, associated with sporadic development of crenulation cleavage, produced culminations and depressions on the F2 fold hinges.