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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: The Sudbury Basin is a non-cylindrical fold basin occupying the central portion of the Sudbury Impact Structure. The impact structure lends itself excellently to explore the structural evolution of continental crust containing a circular region of long-term weakness. In a series of scaled analogue experiments various model crustal configurations were shortened horizontally at a constant rate. In mechanically weakened crust, model basins formed that mimic several first-order structural characteristics of the Sudbury Basin: (1) asymmetric, non-cylindrical folding of the Basin, (2) structures indicating concentric shortening around lateral basin termini and (3) the presence of a zone of strain concentration near the hinge zones of model basins. Geometrically and kinematically this zone corresponds to the South Range Shear Zone of the Sudbury Basin. According to our experiments, this shear zone is a direct mechanical consequence of basin formation, rather than the result of thrusting following folding. Overall, the models highlight the structurally anomalous character of the Sudbury Basin within the Paleoproterozoic Eastern Penokean Orogen. In particular, our models suggest that the Basin formed by pure shear thickening of crust, whereas transpressive deformation prevailed elsewhere in the orogen. The model basin is deformed by thickening and non-cylindrical synformal buckling, while conjugate transpressive shear zones propagated away from its lateral tips. This is consistent with pure shear deformation of a weak circular inclusion in a strong matrix. The models suggest that the Sudbury Basin formed as a consequence of long-term weakening of the upper crust by meteorite impact. [ABSTRACT FROM AUTHOR]