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Orogenic curvature is a ubiquitous feature of mountain belts, and the plate tectonic and geodynamic setting responsible for the development of curved orogens is a subject of debate. In order to distinguish between different models of orocline formation it is necessary to tightly constrain the absolute timing of oroclinal development. However, determining the absolute (isotopic) timing of oroclinal bending is challenging. The most direct method available to constrain the time interval during which an orocline formed is to obtain absolute age constraints on fabrics generated within syn-orocline strike–slip shear zones that accommodated rotations around a vertical axis during the development of crustal or lithospheric scale orogenic curvature. Here we present a data set of 40 Ar/ 39 Ar ages obtained from five shear-zones, some of which display curved traces parallel to the Cantabrian Orocline structural grain in NW Iberia and are interpreted to have been generated coevally to it. The 40 Ar/ 39 Ar ages were obtained by laser-rastering induced step-heating on single muscovite crystals that grew synkinematically during shearing. All five samples yielded ages that cluster tightly at 308 ± 3 Ma providing direct evidence for the age of oroclinal bending. This age is consistent with the age constraints obtained from paleomagnetic and structural data that place the onset of oroclinal bending in Moscovian times. Our 40 Ar/ 39 Ar age determinations therefore provide a reliable absolute constraint on the age of buckling of the Variscan orogen around a vertical axis and provide further evidence that oroclinal bending is unrelated to Variscan convergence/collision or the subsequent extensional collapse of the mountain belt. [ABSTRACT FROM AUTHOR]

Abstract: Deformation structures differing in style and orientation within a given terrane are often attributed to distinct tectono-metamorphic events. However, mechanical heterogeneities may locally cause strain perturbations that can have a profound effect on the geometry of such structures. Here we document highly variable orientations of planar structures, metre- to kilometre-scale folds and shortening directions inferred from brittle fault analysis within the synclinal and layered Sudbury Igneous Complex (SIC) and its Huronian host rocks. NW–SE shortening during Palaeoproterozoic deformation led to the formation of planar structures in the host rocks that are parallel to the NE–SW striking contact of the southern SIC. During deformation, local contact-parallel shortening became more important than regional NW–SE shortening and generated contact-orthogonal planar mineral fabrics and folds. Local contact-parallel shortening is attributed to the shape change of the southern SIC from a convex outward to a concave inward curvi-planar geometry. Contact-parallel shortening accounts for the formation of a previously unidentified kilometre-scale buckle fold in the SIC and respective axial–planar mineral fabrics in Huronian host rocks. This buckle fold shares similar structural characteristics with known higher-order buckle folds of the eastern SIC. We suggest that non-cylindrical buckling and associated mineral fabric development accommodated the shape change of the SIC. This resulted in mutually perpendicular fabric orientations, which are compatible with overall NW–SE shortening. The original shape of the SIC seems to have had a profound influence particularly on the geometry of metamorphic foliations. Our structural analysis supports earlier structural studies advocating that highly discordant planar strain fabrics can be generated by local strain perturbations near igneous sheets under uniform regional shortening. [Copyright &y& Elsevier]

A deflection of the fault controlled southwestern coastline of Vancouver Island suggests the presence of a minor orocline, with a Southern Crustal Block (south of Barkley Sound–Alberni Inlet) rotated 20° counterclockwise relative to a Northern Fixed Crustal Block about a pole of rotation located northeast of Port Alberni. In this paper two models of orocline development, one of pure block rotation and one of pure bending, are proposed. The predictions of these models are tested against available geological maps, structural orientation data, identified regions of extension and contraction, and paleomagnetic data. Structural orientation and paleomagnetic data are consistent with 18° of post-Late Cretaceous counter clockwise rotation of the Southern Crustal Block relative to the Northern Fixed Crustal Block. A southward increase in the magnitude of rotation evident in the structural orientation data argues for a model of bending. Both bending and block rotation models predict the development of a zone of contraction along the northeast margin of the Southern Crustal Block, coincident with the location of the Eocene Cowichan fold-and-thrust belt, that diminishes northward toward the pole of rotation. As predicted, the fold-and-thrust belt is characterized by a northerly decrease in the amount of shortening, from >30% at the south end of the thrust belt, to 0% shortening north of Port Alberni. The northerly decrease in shortening is complemented by a north to south change in structural style from cylindrical to conical folds, and finally to planar, undeformed strata. The model of block rotation predicts the presence of a zone of extension extending southwest from the zone of rotation, coincident with the location of Eocene extensional structures within Barkley Sound and with horst and graben structures in the offshore Eocene to Miocene Tofino basin. Extension is less than predicted by a model of pure block rotation and suggests that much of the oroclinal rotation was accommodated by bending. Timing constraints indicate that orocline development was coeval with, and resulted from, the Eocene accretion of seamounts of the Crescent terrane. These findings demonstrate that oroclinal orogeny, the buckling of a linear crustal beam about vertical axes of rotation, can significantly impact the geometry, structure and character of an orogenic belt, even where the buckles are minor (<20° of rotation). [Copyright &y& Elsevier]