7.3 Timing of extensional shearing along the ZSZ
Several arguments indicate close spatial and chronological relations between extension along the ZSZ, anatexis in the Migmatite Zone, and leucogranite intrusions. The coherent metamorphic field gradient defined by the P-T results (fig. 5.21) shows that the metamorphic evolution in the HHCS is the consequence of an underthrusting event which preceded the activation of the ZSZ. These results also indicate that peak metamorphic conditions in the migmatitic zone during underthrusting were close to but not necessarily sufficient for partial melting of these rocks through dehydration melting of muscovite.
The metamorphic peak conditions in the Migmatite Zone, as well as in the lower ZSZ, were followed by nearly isothermal decompression implying an exhumation faster than the thermal relaxation of the perturbed isotherms (fig. 5.22). Such a retrograde P-T path is characteristic of tectonic denudation processes (e.g. Ruppel et al., 1988) and is the consequence of the very rapid exhumation of the HHCS. Production of leucogranitic melts thus started once the decompression path of the rocks from the migmatitic zone crossed the muscovite dehydration-melting solidi.
Geochronological results indicate that the leucogranites from the Intrusion Complex cooled
below T 725± 25°C at 22.2 ± 0.2 Ma (fig. 6.14). On the basis of the P-T results (fig. 5.22), we conclude that the production of leucogranitic melts was preceded by a nearly isobaric decompression which we interpret as the consequence of a rapid exhumation. This interpretation implies that cooling at 22.2 ± 0.2 Ma recorded an already ongoing stage of extension. The onset of extension (and leucogranite production) was however probably not significantly older than 22.2 Ma, although it cannot be constrained more precisely.
The magmas produced in the Migmatite Zone rose through a swarm of feeder dikes which coalesced below the ZSZ to form the flattened, sill-like plutons of the Intrusion Complex. These relations suggest that intrusion took place through fracture propagation and that the ZSZ acted as a mechanical and/or thermal discontinuity which hindered this propagation and further magma ascent (e.g. Clemens & Mawer, 1992). Several generations of leucogranitic dikes nevertheless intruded the base of the ZSZ. Most of these dikes are strongly deformed, but the presence of some undeformed dikes crosscutting the base of the shear zone clearly indicates that the leucogranite intrusion outlasted the main phase of ductile deformation.
The undeformed leucogranite dikes intruding the base of the ZSZ cooled below the closure temperature for Ar diffusion in muscovite (T 420 ± 50 °C) between 19.8 ± 0.1 and 19.3 ± 0.1 Ma (Table 6.3). These data indicate that the main ductile deformation along the ZSZ ceased before 19.8 ± 0.1 Ma. It is nevertheless likely that some brittle extension in the ZSZ hanging wall outlasted the ductile shearing (e.g. Sarchu Fault).
Consequently, our geochronological data suggest that ductile shearing along the ZSZ occurred between 22.2 ± 0.2 Ma and 19.8 ± 0.1 Ma and lasted not much more than 2.4 ± 0.2 My.