The ASRR did not follow a small circle of the Euler rotation pole between the Indochina and South China blocks. Such geometry induced transpression along the NW and transtension along the SE portions of the shear zone. The neutral point corresponding to the place where the fault zone aligns with a small circle was the only place where motion was purely strike-slip. Rocks originally located within the Indochina block along of the NW portion of the fault zone will thus first deform in a transpressional regime, then in pure strike-slip near the neutral point, before entering the transtensional domain. In this zone, extension induces normal faulting and thus denudation and rapid cooling of the footwall of the fault zone. The apparent rate of progression of cooling along strike would thus correspond to the rate at which rocks located on the Indochina block pass through the neutral zone and enter the transtensional zone (Fig. 20-II). The neutral point was located between the AilaoShan and DCS ranges, in the "Midu metamorphic gap", where denudation has been minimal. The parts of the shear zone located farther north (XLS, DCS) would not have been exhumed by transtension but by local mechanisms. In the SW part (Ailao Shan, DNCV) cooling (I) resulted from diachronic regional denudation in a transtensive regime. The induced cooling diachronism observed in the Ailao Shan suggests that left-lateral deformation took place at a rate of 4 to 5 cm/yr from 27 Ma until 17 Ma (Fig. 20-I). DNCV rocks, belonging to the South China block, always stayed in a transtensive regime and do not show cooling diachronism.
This zipper tectonic model is in very good agreement with the ASRR kinematics that have been inferred from the kinematics of South China Sea seafloor spreading (Briais et al., 1993) (Fig. 21). This confirms the causal link between large-scale continental strike-slip faulting and marginal basin opening. On figure 21 we propose structural reconstruction of SE Asia at 32 Ma (before sea floor spreading in the SCS) and at 16 Ma (end of the extrusion process) (Leloup et al., 1995; 2001).
