The Evolution of Back-Arc Extension Driven by the Interference of Different Subduction/Collisional Systems

Abstract:

Extensional back-arc basins form in the hinterland of orogens by collapsing the upper plate during the rapid roll-back of subduction systems. During convergence of major tectonic plates, a number of rather local roll-back systems may develop on a spatially restricted area, inferring the possibility of back-arc extension driven by different slabs taking place in the same area. This is the case of the many instances of rapid roll-back subductions that dominated the recent evolution of the Mediterranean and SE Asia orogenic systems. The extension may take place far at the interior of the upper plate, as is the case in various segments of the Carpathians or in the core of the SE Asian domain, but in most cases of the Dinarides, Apennines, Rif-Betic or Hellenides it take place superposed or far into the foreland when compared with the position of oceanic suture zones. Such observations are often difficult to reconciliate with the classical position of a back-arc basin relative to the location of volcanic arcs. The geometry of such back-arc basins is asymmetric, deposition of sediments being associated with rapid exhumation of detachments footwalls, often reactivating nappe contacts or suture zones, as observed in the Aegean, Pannonian or Alboran domain. In all these orogenic systems, the partly coeval shortening has gradually duplicated crustal blocks from the lower plate and shifted the subduction zone far towards the foreland without any significant formation of retro-wedges. The moments of interaction between the roll-back of different slabs in the same back-arc basin can be detected by deviations from the general asymmetry, observed in the regional geometry of the sedimentary fill. Quantifying the extension driven by the interaction between these different roll-back systems relies on coupling of kinematics of the back-arc basin with the evolution of the driving subduction systems. Such observations infer the need of understanding the coupling between orogenic and back-arc processes at various spatial scales in order to understand the driving mechanisms and associated sedimentary basins dynamics.