ANALOGUE MODELING OF OROCLINES: BUCKLING OF RIBBON CONTINENTS AND MAGMATIC ARCS

We use scaled 3-D thermo-mechanical analogue models to investigate the formation of oroclines (originally linear orogens now curved in map view by buckling about vertical axes). The experimental setup consists of a tank of water (the asthenosphere) on top of which rest hydrocarbon plates (the lithosphere) with strain softening behaviours and thermo-dependent elasto-plastic properties. An electric heating element below and 4 infrared lights above produce a constant vertical (geo) thermal gradient in the plates. A horizontal piston drives constant plate motion and gives rise to a compressional stress regime. Geometric, kinematic and dynamic variables are calibrated in accordance with a set of scaling laws and proper plates’ composition.

Our results suggest that oroclinal buckling involves the entire lithosphere and cannot be confined to the crust only. A wide range of syn-oroclinal structures developed during buckling, including thin- to thick-skinned thrust belts, transform faults and extensional structures, as well as extensional basins and subduction zones in the lithosphere adjacent to the ribbons. During oroclinal buckling, a thrust belt forms upon complete closure of the interlimb region and is attributable to the trailing orocline limb overthrusting the leading orocline limb. An analogous syn-oroclinal thrust system characterizes the Central Iberian Orocline (CIO) of the Variscan orogen in Iberia where the north limb of the convex to the west orocline exposes recumbent north-verging folds while the overriding south limb bears upright to gently north-verging folds. Our results imply that these structures developed during final closure of the CIO, and indicate that the north- and south- limbs of the CIO constitute the leading- and trailing-limbs, respectively, of an orocline that formed by overall northward translation. High geothermal gradient experiments, modeling magmatic arcs, yield late stage transform faults that bisect the buckling arcs. This outcome is analogous to the Panama Canal fault zone that severs the buckled Panamanian Isthmus. The hinge zones of modeled oroclines are the sites of subduction initiation, similar to initiation of Caribbean plate subduction beneath the convex to the north, North Panamanian orocline, and Calabrian subduction beneath the Calabrian orocline of Sicily.