ON THE ORIGIN OF ARCUATE FOLDED BELTS AND BACK-ARC BASINS

We present a new hypothesis to explain the origin of extensional basins (e.g. back-arc basins) in convergent settings. In our model the basins opened as mega continental tension gashes parallel to the convergence direction. The gash opens along pre-existing, deep, parallel and steep dipping faults which separate lithospheric units with different physical properties. These faults are typically normal or strike-slip faults; at passive margins the main faults are extensional and parallel to the margin. In recent years, research into convergent boundaries has shown that, in addition to thrust faults, synorogenic normal faults are common in the hinterland of orogenic belts. Thus, long, plastic/elastic units, bounded by faults, exist at the continental margins or close to them. If plate convergence is parallel to these faults, the plastic/elastic fault bounded units open at right angles to the convergence vector, adopting an arcuate shape, with thrusting in front of the bowed-out units and an extensional basin opening between the separating units. Initially, extension is due to upper crustal collapse, which fills the void caused by arc migration. Arc migration pushes the subducting (and denser) plates horizontally, and this contributes to the subduction roll back. Asthenospheric material rises below the extensional basin and is shortened and expelled laterally owing to progressive convergence, thereby helping subduction roll back. Extension in the central basin and subduction could be asymmetric given that they are controlled by the physical properties (e.g. density, thickness, length, elasticity) of the former fault-bounded crustal units.