Discussion and conclusion

One can easily recognise in the thermo-mechanical experiment in Fig. 5 evolution of the continental subduction and exhumation corresponding to the low-compression regime obtained in the isothermal experiments (Chemenda et al., 1996). As in the previous experiments, starting from some depth of continental subduction, the continental crust undergoes failure and buoyancy driven uplift along the interplate zone followed by the mantle layer break off. The thermo-mechanical model, however, provides new important insights into this process. Crustal failure and uplift have proved to be closely related to the delamination of the subducting crust and the mantle. The delamination is caused by the pull force generated by the subducted oceanic lithosphere and the continental mantle (both are denser than the asthenosphere, see Table 1) and occurs due to the large ductile deformation of the crust, especially of the lower crust as well as of the upper crust subducted into the asthenosphere (to ~100 km-depth). These warmed and weakened units flow up under the upper crust segment located between the overriding and subducting plates, being driven by the buoyancy force. The upper crust griped between the overriding and subducting plates undergoes much smaller internal deformation in spite of the fact that, deeper than a few tens of kilometres, it is also very weak and can flow under small differential stress. The upper crust segment finally fails at about 40 km-depth, forming a coherent rising slice which reaches about 20 km-depth. Overthrusting and uplift of this slice as well as the upward flow of a deeper subducted crust correspond to the delamination of the crust and the mantle. In the presented experiment this process was stopped by the break off which removes the pull force, the driving force of the delamination, but in other experiments conducted under slightly different conditions we obtained a total separation of the crustal and mantle layers. After the break off both the delamination and delamination-related rise of the crust were stopped.

Figure 5. Drawings of the experimental photos in Fig. 3.

Table 1. Parameter values adopted for the model and nature. ss, sc and sl are the average yield limits under normal loading of the sedimentary cover, the crust, and the mantle, respectively; Hs, Hc and Hl, are the thickness of the sedimentary cover, the crust, and the mantle, respectively. rs, rc, rl and ra are the densities of the sedimentary cover, the crust, the mantle and the asthenosphere. V is the convergence rate, t is the time and k is the thermal diffusivity of the lithosphere.

The thermo-mechanical experiments reveal also very interesting burial/exhumation evolution of the sedimentary cover. The sediments of the continental margin are dragged to the overriding plate base, are partially accreted (underplated) at the lower part of the interplate zone (at 60-70 km-depth) and partially flow under the overriding plate base, being pushed by the crust (Fig. 5b). The underplated sediments remain at their place until the beginning of the delamination and formation of the upper crustal slice 2 (Fig. 5d). During the delamination, the coupling between the crust and the mantle reduces and the crust is not pulled down anymore by the dense mantle layer. Therefore the pressure between the crust and the overriding plate increases along the interplate zone starting form its deepest part as the delamination propagates from the overriding plate base upwards. The increasing pressure squeezes the underplated sediments of the continental margin. They are extruded upward, overtaking the rising upper crustal slice (Figs. 4 and 5e). After this rapid exhumation a small volume of the continental margin cover subducted to ~70 km-depth reaches 15-20 km-depth and finds itself above the crust exhumed from much smaller depth (ca. 40 km). The increased interplate pressure makes it difficult for new portion of the sedimentary cover to enter the interplate zone: they are scraped off and accreted in front of subduction zone (Figs. 3h and 5e). The continental margin sediments entered the asthenosphere flow up with the deeply subducted continental crust under the overriding plate into the arc area (Fig. 5d) where they can be eventually exhumed.

In the presented experiments we were not able to obtain the exhumation from depths exceeding the overriding plate thickness, i.e. 60-70 km (or probably maximum ~100 km), while in reality this depth can reach ~150 km as for example, in Dabie Shan and Kazahstan (Hacker and Peacock, 1994; Ernst and Liou, 1999). The reason is a very low crustal strength at these depths, allowing the crust to flow when it is not limited by more rigid units (overriding and subducting plates). A possible way of obtaining the exhumation of ultra-high pressure (UHP) rocks could be an integration into the modelling of one more element, the fore arc block subduction. Such a block can serve both as a rigid guide for the deep crustal subduction and exhumation, and as a thermal shield protecting the deeply subducted crust from overheating by the mantle (Chemenda et al., 1997, 2001), and thus providing a low temperature under the UHP conditions.

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