Tectonic models for the formation of arc-shaped convergent zones and backarc basins

Four popular tectonic models are discussed that attempt to explain the formation of arc-backarc systems. These systems develop in a convergent setting with shortening in the forearc region, extension in the backarc region, and progressive out-bowing of the arc. The models include the gravitational collapse model, the rollback model, the extrusion tectonics model, and the orogen-parallel compression model. The rollback model can explain the progressive development of most arcs, such as those found in the Western Pacific and the Mediterranean, in combination with backarc extension. Slab rollback ultimately is a consequence of the negative buoyancy of the slab. Collapse models can explain radial thrusting in the foreland and exten-sional deformation in high-standing mountain belts, since these regions involve large potential energy contrasts between mountain range and foreland. However, these models cannot explain the development of Western Pacific and Mediterranean style arc-backarc systems. In such settings, the extending region has a small potential energy, which cannot drive arc formation, fore-arc shortening, and backarc extension. The extrusion tectonics model can explain strike-slip structures such as those observed in the Eastern Alps, Anatolia, and East Asia, but fails to explain backarc extension. The extrusion of a wedge cannot produce shortening at its leading edge contemporaneously with extension in the middle of the wedge. The orogen-parallel compression model cannot explain arc formation and backarc extension, since an orogen does not behave elastically in the plane of the lithosphere at a length scale of ∼≥1000 km.