OROCLINAL OROGENY

Oroclinal orogeny requires the displacement of a lithospheric beam, for instance a magmatic arc, a seamount chain or a ribbon continent. Pinning of the leading tip of the translating beam may then result in buckling about vertical axes of rotation (oroclinal orogeny), much like the scissoring of the cars of a derailing train. The resulting oroclines are thick-skinned, can be of continental-scale, accommodate significant margin-parallel displacement, and require either subduction or substantial crustal thickening along the inside of the buckles, and extension along the outsides. Examples include the (1) Great Alaskan and (2) Southern Vancouver Island oroclinal orogens. (1) The Late Cretaceous Great Alaskan oroclinal orogeny consists of a series of buckles that affected SAYBIA, a >7000 km long ribbon continent that extended south from eastern Siberia and which lay west of autochthonous North America in the Cretaceous. Paleomagnetic data indicate that the southern portions of SAYBIA traveled ~3000 km north between 85 and 50 Ma. Pinning of the northern tip of the northward translating ribbon continent against Siberia resulted in buckling of the northern portion of SAYBIA, with scissored ‘cars’ forming much of Alaska (Johnston, 2001, EPSL 193, 259-272). Oceanic crust caught between the buckles was subducted, giving rise to widespread Late Cretaceous magmatism across northern SAYBIA. Buckling was accommodated by detachment of the crust from its underlying mantle, the fate of which remains unknown. (2) Eocene bending of Southern Vancouver Island gave rise to extension along the outer (western) curve of the orocline forming the Middle- to Late Eocene Tofino basin and Barkley Sound. Contraction along the inner curve of the orocline resulted in the development of the thick-skinned Cowichan fold and thrust belt. Shortening across the fold and thrust belt increases from zero near the axis of the orocline in the north to 40% near the southern end of the fold and thrust belt. Paleomagnetic data from the Late Cretaceous Nanaimo group (Enkin et al., 2001) are consistent with a 20° CCW rotation relative to correlative strata north of the axis of the orocline. Oroclinal orogeny resulted from the end on collision with the Crescent terrane seamounts (Johnston and Acton, Tectonophysics, in review).