ZHIBIN LEI, J. Huw Davies 

Rapid trench retreat, or slab roll-back, is often observed in subduction zones where active and focused back-arc extension develops. However, the correlation between trench retreat rate and back-arc extension has not been rigorously tested. Here we study the correlation by investigating a series of 2-D thermo-mechanical and internally driven numerical models with a mobile overriding plate in set-ups that lead to a wide range of trench retreat rate. The results produced three grades of localised back-arc stretching states as the trench retreat rate increases: i) minor extension with observable but not significant thinning of the thermal lithosphere; ii) rifting extension with hot mantle thermally intruded upward to the surface; iii) new spreading seafloor with thin lithosphere. We find that it takes a minimum trench retreat rate to initiate rifting extension in the back-arc. The extension is driven by the non-uniform basal drag of the trenchward mantle wedge flow due to rapid trench retreat. The results could potentially be used to explain the southward decreasing stretching state along the Lau-Havre-Taupo back-arc system where trench retreat rate decreases southward as well. On the other hand, in the models we find that an older subducting plate leads to faster trench retreat rate, but this correlation only exists before the slab approaches the lower mantle. This matches well with the observation that the subducting plate age is always old (>55 Ma) in subduction zones with back-arc extension. It also explains why there is a poor correlation between the age of subducting plate and trench retreat rate because most slabs have already reached the lower mantle.