General shear extrusion, syn-convergence extension and inverted metamorphism during the Himalayan orogeny: One cause, two consequences?

Two paradoxical geological features of the Himalaya are the synconvergence extension and the inverted metamorphic isograds observed in the crystalline core zone of this orogen. This High Himalayan Crystalline Sequence (HHCS) corresponds to an up to 40 km thick sequence of amphibolite to granulite facies gneiss, bounded by the Main Central Thrust (MCT) at the base, and by the extensional faults of the South Tibetan Detachment System (STDS) at the top. The HHCS systematically shows an inverted metamorphic zonation, generally characterized by a gradual super-position of garnet, staurolite, kyanite, sillimanite + muscovite and sillimanite + K-feldspar isograds, from the base to the top of the unit. The parallelism between these inverted isograds and the MCT at regional scale suggests a close genetic relation between inverted metamorphism and deformation during exhumation of the HHCS. Yet, quantitative thermal models suggest that although peak isotherms in the hanging wall of the MCT are readily bent toward the foreland during thrusting, they are not necessarily inverted. On the other hand, kinematic models proposing an inversion of isograds in the HHCS as a consequence of a late to post-metamorphic deformation through folding or ideal simple shear are not compatible with the general lack of inverted pressure field gradient across the sequence. Recently, Grujic et al. (1996) demonstrated the coexistence of both simple shear and pure shear, in other words a general non-coaxial flow, during the ductile deformation of the HHCS in the Himalaya of Bhutan, and these authors proposed a qualitative channel flow model for a ductile extrusion of this unit. On the basis of new structural and P-T constraints from the Sutlej section, we developed a quantitative kinematic model for a general shear extrusion of the HHCS in the NW Himalaya. Such a general shear extrusion is consistent with: (1) the general lack of inverted peak pressure field gradient across the HHCS, despite the intense non-coaxial deformation systematically observed across this unit; (2) kinematic indicators demonstrating a ductile deformation combining simple shear and pure shear in the HHCS (Grujic et al., 1996); Grasemann, Fritz and Vannay, 1999); (3) structural and P-T constraints indicating that the HHCS, that corresponds to the subducted sedimentary cover of the Indian plate, behaved as a low viscosity rock sequence during the syn-MCT exhumation, as a consequence of the pre-MCT high-grade metamorphism and partial melting at peak conditions between 600 and 750°C (e.g. De`zes et al., 1999; Vannay, Sharp, Grasemann, 1999); and (4) seismic data indicating that the HHCS has the geometry of a large-scale orogenic wedge, bounded by the MCT and STDS converging 30-40 km beneath the present-day surface (Hauck et al., 1998). Moreover, the general shear extrusion model we propose can predict: (1) inverted isograds in the HHCS, as a consequence of the simple shear component of the ductile flow inducing a foreland-directed rotation of peak isograde; (2) a syn-convergence extension (STDS) compatible with a geodynamic compressional setting and reflected by a compressional extensional detachment; (3) a syn-convergence extension coeval with underthrusting along the MCT during Early Miocene (e.g. De`zes et al., 1999; Hodges, 2000), because this extension is a direct consequence of the deformation induced in the HHCS by the subducting Indian plate; (4) a rapid syn-convergence extension in a few My, as constrained by geochronological and structural data (e.g. De`zes et al., 1999); (5) a syn-convergence extensional detachment converging at depth toward the MCT-Main Himalayan Thrust contractional structures (Hauck et al., 1998), because this detachment initiates at this convergence point as a stretching fault (Means, 1989) recording a gradually increasing slip toward the surface; and (6) a syn-convergence...