Conclusions

Several back-arc basins display a geometry and internal structure that is asymmetric, i.e. where the amount of extension increases from one end of the arc to the other. We presented a simple model to explain this geometry, where the opening is controlled by rollback of the hinge-line of the subducting lithosphere. The increase in extension is explained by the increase in retreat velocity of the subducting slab along the trench axis. This increase in velocity can be caused by an increase in lithospheric density along the trench axis and/or a vertical tear in the slab at the more rapidly retreating side of the retreating slab. We have presented the results of 3-dimensional analogue models to simulate asymmetrical back-arc extension of an overriding lithosphere. In these models the initial rheology has been varied. The results show that with increasing lithospheric brittle to viscous strength (BS/VS), the fault density decreases, while the asymmetry in deformation pattern in the back-arc region increases. It is also shown that with decreasing brittle strength to buoyancy force ratio (BS/BF) the total area of surface deformation increases. The models and experimental results can be compared with several arc - back-arc systems, which display a relatively large amount of asymmetry along the arc and in the back-arc region (e.g. Tonga Arc, Kuril Arc, New Hebrides Arc, Ryukyu arc) but a varying style of tectonic deformation. These differences are mainly the result of the stage of opening up of the back-arc basin and differences in rheology of the overriding lithosphere.
The analogue results indicate that with an initially buoyant and weak overriding plate rheology, deformation can propagate inboard far from the retreating boundary, where the amount of propagation is comparable to the width of the retreating boundary. We make the tentative proposition that extension and strike-slip faulting observed in East Asia do not necessarily have to be related to escape tectonics due to India-Eurasia collision. These structures could also be explained by tensional deviatoric stresses along the East Asian active margin due to slab retreat towards the east-southeast. During the Oligocene to Middle Miocene, back-arc extension along the East Asian margin took place along a continuous zone of more than 6000 km in length, stretching from the South China Sea in the southwest to the Sea of Okhotsk in the northeast. This could have led to extension in the overriding plate at a distance as far as 3000-4000 km away from the active margin, such as extension in the Shanxi graben and the Baikal rift.

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