The maximal values of Po/σc are ~1.4 to the left of the shear zone (Figure 3c; X/hc ~ −1.5) and ~0.7 inside the shear zone (Figure 3c; X/hc ~ 0.3). The σc is typical of the order of 1 GPa (e.g., 2800 kg m−3 × 9.81 m s−2 × 35 km), and Po can reach such values also if τII at the same location is significantly smaller (Figure 3). The maximal values of T/Tc in the crust inside the shear zone are ~1.4, providing for initial temperatures at the Moho Tc = 500–600°C values of T = 700–840°C, which indicate a temperature increase due to viscous heating in agreement with previously published studies [e.g., Burg and Gerya, 2005]. Such increased values of T and Po can affect the P-T-time trajectories of deformed rocks considerably. The results have, therefore, considerable implications for the geodynamic interpretation of HP-UHP rocks found in tectonic nappes because many studies assume that P is always identical to the lithostatic pressure and that Po is negligible [e.g., Jolivet et al., 2003]. A typical argument against tectonic overpressure is that many rock units in which HP-UHP rocks are found are mechanically weak (i.e., low effective viscosity and τII) and, therefore, significant values of Po are not possible [e.g., Brace et al., 1970; Schreyer, 1995]. Several studies [e.g., Gerya et al., 2008; Li et al., 2010] presented numerical results where significant Po (up to 100% of lithostatic values) is common in the lithosphere for subduction scenarios. However, the significant Po in these simulations is indeed restricted to strong rocks, while weak rocks usually exhibit low Po. In particular, rock samples that returned to the near surface from great depth within the weak subduction channel do not record significant Po [Li et al., 2010]. These numerical studies, therefore, support the abovementioned argument that weak rocks do not record significant Po. The results presented here also show that no significant Po occurs in the mantle part of the shear zone (Figure 2b). Our results do, however, restrict the context of applicability of this argument and show that during continental collision, significant Po can occur in weak crustal shear zones (i.e., rocks exhibiting τII ≪ Po). Many natural nappes are considered as result of thrust-type ductile shearing in crustal rocks. In the central Alps, such nappes often preserve their original Mesozoic-Tertiary sedimentary cover and exhibit considerable lithological consistency [e.g., Steck, 1990; Nagel, 2008]. Rooting of these crustal nappes in a weak subduction channel in the deep mantle is entirely due to conversion of pressure to depth based on the assumption of negligible Po that, as we show here, may not be universally applicable to all weak shear zones. A considerably simpler interpretation of high pressure recorded by crustal thrust-type weak shear zones is due to development of significant Po in order to satisfy the horizontal force balance during the continental collision.