Abstract Orientated carbonate (calcite twinning strains; n = 78 with 2414 twin measurements) and quartzites (finite strains; n = 15) were collected around Gondwana to study the deformational history associated with the amalgamation of the supercontinent. The Buzios orogen (545–500 Ma), within interior Gondwana, records the high-grade collisional orogen between the São Francisco Craton (Brazil) and the Congo–Angola Craton (Angola and Namibia), and twinning strains in calc-silicates record a SE–NW shortening fabric parallel to the thrust transport. Along Gondwana's southern margin, the Saldanian–Ross–Delamerian orogen (590–480 Ma) is marked by a regional unconformity that cuts into deformed Neoproterozoic–Ordovician sedimentary rocks and associated intrusions. Cambrian carbonate is preserved in the central part of the southern Gondwana margin, namely in the Kango Inlier of the Cape Fold Belt and the Ellsworth, Pensacola and Transantarctic mountains. Paleozoic carbonate is not preserved in the Ventana Mountains in Argentina, in the Falkland Islands/Islas Malvinas or in Tasmania. Twinning strains in these Cambrian carbonate strata and synorogenic veins record a complex, overprinted deformation history with no stable foreland strain reference. The Kurgiakh orogen (490 Ma) along Gondwana's northern margin is also defined by a regional Ordovician unconformity throughout the Himalaya; these rocks record a mix of layer-parallel and layer-normal twinning strains with a likely Himalayan (40 Ma) strain overprint and no autochthonous foreland strain site. Conversely, the Gondwanide orogen (250 Ma) along Gondwana's southern margin has three foreland (autochthonous) sites for comparison with 59 allochthonous thrust-belt strain analyses. From west to east, these include: finite strains from Devonian quartzite preserve a layer-parallel shortening (LPS) strain rotated clockwise in the Ventana Mountains of Argentina; frontal (calcite twins) and internal (quartzite strains) samples in the Cape Fold Belt preserve a LPS fabric that is rotated clockwise from the autochthonous north–south horizontal shortening in the foreland strain site; Falkland Devonian quartzite shows the same clockwise rotation of the LPS fabric; and Permian limestone and veins in Tasmania record a thrust transport-parallel LPS fabric. Early amalgamation of Gondwana (Ordovician) is preserved by local layer-parallel and layer-normal strain without evidence of far-field deformation, whereas the Gondwanide orogen (Permian) is dominated by layer-parallel shortening, locally rotated by dextral shear along the margin, that propagated across the supercontinent.

ABSTRACT The North American Cordillera is generally interpreted as a result of the long-lived, east-dipping subduction at the western margin of the North American plate. However, the east-dipping subduction seems problematic for explaining some of the geological features in the Cordillera such as large volume back-arc magmatism. Recent studies suggested that westward subduction of a now-consumed oceanic plate during the Cretaceous could explain these debated geological features. The evidence includes petrological and geochemical variations in magmatism, the presence of ophiolite that indicates tectonic sutures between the Cordillera and Craton, and seismic tomography images showing high-velocity bodies within the underlying convecting mantle that are interpreted as slab remnants from the westward subduction. Here we use 2-D upper mantle-scale numerical models to investigate the dynamics associated with westward subduction and Cordillera-Craton collision. The models demonstrate the controls on slab breakoff (remnant) following collision including: (1) oceanic and continental mantle lithosphere strength, (2) variations in density (eclogitization of continental lower crust and cratonic mantle lithosphere density), and (3) convergence rate. Our preferred model has a relatively weak mantle lithosphere, eclogitization of the lower continental crust, cratonic mantle lithosphere density of 3250 kg/m 3 , and a convergence rate of 5 cm/yr. It shows that collision and slab breakoff result in an ~2 km increase in surface elevation of the Cordilleran region west of the suture as the dense oceanic plate detaches. The surface also shows a foreland geometry that extends >1000 km east of the suture with ~4 km of subsidence relative to the adjacent Cordillera.

Abstract Recent structural studies of the Apennines and the Calabrian orocline and a compilation of structural, stratigraphic, GPS and palaeomagnetic data from the central and western Mediterranean region show that beginning in the Late Miocene a N–S trending ribbon continent that had been previously deformed, and which we now recognize as the Apennine–Sicilian thrust belt, buckled eastward in response to northward movement of Africa relative to stable Europe. A simple geometric model is consistent with available data and shows how eastward buckling of an originally north–south continental beam explains: (1) opening of the Tyrrhenian Sea basin from 7–2 Ma, at which point sea-floor spreading ceases and the basin begins to shrink by southward subduction beneath Sicily; (2) the coeval development of an east-verging fold-and-thrust belt along the length of the Apennine–Sicilian belt in response to overthrusting of the autochthon to the east, followed by extension beginning at 1 Ma as the Apennine portion of the beam begins to retreat to the SW; and (3) subduction of continental and oceanic lithosphere east of the buckling beam into a trench that migrates eastward through time due to ‘push back’ by the buckling upper plate.

The origin of map-view bends of orogenic belts and arcs remains enigmatic. Here I summarize geological evidence indicating that a bend of the northern end of a ribbon continent extending north from the Northland Peninsula, New Zealand, through New Caledonia and the Loyalty Islands and into the submarine d’Entrecasteaux ridge (the NNNCd’E ribbon continent) is an orocline that has formed as a result of oroclinal orogeny (buckling about vertical axes of rotation due to pinning of the leading edge of a migrating lithospheric beam). An analogue model is used to investigate the geometric relationship between orocline development, the rotation of the Vanuatu–New Hebrides arc, and the origin of the North Fiji basin. The NNNCd’E ribbon continent terminates to the northeast in the Vanuatu–New Hebrides arc. The asymmetric, triangular, northwest-tapering North Fiji lies northeast of (behind) the arc. Paleomagnetic, geological, and geodetic data imply that since 10 Ma the Vanuatu–New Hebrides arc has rotated ~60° clockwise about a pole of rotation located at its northwest end, opening the North Fiji basin. The analogue model demonstrates that the arc was forced to rotate clockwise due to its southward advance being impeded by the NNNCd’E ribbon continent. In this model, the ribbon continent originated as a linear, north-trending feature that ended in the arc, just west of its midpoint. Southward advance of the arc buckled the ribbon continent, giving rise to the orocline. Buckling forced the arc to rotate clockwise, opening the asymmetric North Fiji basin. The lithospheric-scale of buckling is consistent with orocline formation controlling the clockwise rotation of the Vanuatu–New Hebrides arc and formation of the North Fiji basin. Buckling of the NNNCd’E ribbon continent against the migrating Vanuatu–New Hebrides arc explains the curved ribbon continent, the clockwise rotation of the arc, and the origin of the North Fiji basin. This ongoing oroclinal orogeny provides us with an opportunity to further understand the processes responsible for and involved in the buckling of lithospheric beams and to refine interpretations of ancient orogens that are thought to be the products of oroclinal orogeny.