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1 Département des Sciences de la Terre, Université de Cergy-Pontoise, 8 Le Campus, 95011 Cergy-Pontoise Cedex, France, URA CNRS 1759
2 Dipartimento di Scienze Geologiche, University of Rome Tre, Largo R. Murialdo 1, Roma 00154, Italy
3 Département de Géotectonique, Université Pierre et Marie Curie, T 26-0 El, 4 Place jussieu, 75252 Paris cedex 05, France, URA CNRS 1759
4 Shell International Exploration and Production BV, PO Box 162, 2501 AN The Hague, The Netherlands
Three classical examples of marginal basins are explored to show the respective contributions of body forces and far-field stresses to the extension mechanism. Extensional stresses can be provided by: (1) the slab-pull force, which induces a retreat of the slab and which originates in the density contrast between the subducting slab (oceanic or continental) and the asthenosphere; (2) by lateral density contrasts within the crust (due to crustal thickening), which induce crustal spreading; and (3) by far-field stresses due to intra-continental deformation (continent-continent collision, for example). Slab pull is probably the most efficient extensional force to provide but its effects are modulated by the contributions of the two other forces. The Japan Sea opened along the eastern margin of the Eurasian continent because extensional boundary conditions were provided by the retreating Pacific subduction zone. The opening stopped as soon as the central Japan triple junction was established in its present position which resulted in a more efficient coupling between the Pacific and Eurasian Plates through the Philippine Sea Plate. The geometry of opening was further controlled by large-scale dextral strike-slip faults that run oblique to the subduction zone along >2500 km, and which are far-field effects of the India-Asia collision. The Northern Tyrrhenian Sea opened because of the retreat of the Adriatic continental slab. The strong slab-pull force is probably due to phase changes within the subducting lower crust. Crustal delamination leads to a warm lower crust which localizes the extensional strain. This extending domain migrated with time from west to east as the delamination and slab retreat proceeded. Upper crustal units incorporated in the Apennines accretionary wedge were later exhumed in the collapsing back-arc domain where their deformation and P-T history can now be observed. A similar history with an outward migration can be proposed for the Aegean Sea with, however, a stronger influence of crustal collapse over a larger domain. Here too continental collision (the Arabia-Eurasia collision) controlled the geometry of opening through the westward propogation of the North Anatolian Fault.
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