7.4 Discussion

The results of the present study show that the ZSZ accommodated at least 12 ± 3 km of vertical displacement, which corresponds to a net slip of 35 ± 9 km for the present-day dip of the shear zone. We consider these values as strict minima because several arguments strongly suggest that the ZSZ might have accommodated as much as 100 Km net slip. Geochronological data demonstrate that the main ductile shearing ceased before 19.8 Ma and that the leucogranite in the ZSZ foot wall cooled through the U-Pb closure temperature for monazite at 22.2 ± 0.2 Ma, which is interpreted as a close minimum age for the onset of the main extensional movement.

Considering these data, the slip rates along the ZSZ can be estimated to vary between 1.4 cm/y and 4.2 cm/y. Similarly, the vertical exhumation rate of the HHCS can be estimated between 5mm/y and 8.8 mm/y.

Although the STDS is not a single continuous structure along the Himalayan range, a comparison of available data reveals several first order similarities. In the Annapurna - Manaslu massifs of central Nepal, ~ 800 km to the ESE of Zanskar, several segments of the STDS appear to have been active during the 24 - 21 Ma time interval : Deorali detachment 22.5 Ma, Hodges et al. (1996); Dudh Khola - Chame detachment = 24 - 21 Ma, Coleman (1996); Manaslu detachment 22 Ma, Guillot et al. (1994) and Harrison et al. (1995). In the Mt. Everest massif, ~ 1000 km to the ESE of Zanskar, a minimum displacement of 34 km occurred along the Qomolangma detachment
(Burchfiel et al., 1992). According to Hodges et al. (1992), this detachment was active between 22 and 19 Ma, although new data suggest a younger movement at 16.6 Ma (K.V. Hodges, personal communication, 1997). Such younger movements, as well as multi-stage extension, have been determined for other strands of the STDS, such as in the Annapurna massif (Machhapuchhare detachment 18.5 Ma, Hodges et al., 1996), in Central Nepal (Shisha Pangma detachment 17.3 Ma, Searle et al., 1997), or in Bhutan (Khula Kangri detachment 12.5 Ma, Edwards and Harrison, 1997). It is also likely that the brittle normal faulting observed in the hanging wall of the ZSZ corresponds to such late movements.

Collectively, these results indicate that synorogenic extension along segments of the STDS initiated during the earliest Miocene and that this system of detachments remained active for several millions of years until Middle to Late Miocene. This protracted extensional evolution was thus broadly coeval with thrusting of the HHCS along the MCT between the earliest Miocene ( 23 Ma; Frank et al., 1977; Hubbard and Harrison, 1989; Harrison et al., 1995; Hodges et al., 1996) and the Late Miocene ( 6 Ma; Harrison et al., 1997). These data indicate that combined thrusting at the base, and extension at the top of the HHCS, assisted the exhumation of this high-grade metamorphic sequence in the core of the Himalayan orogen.