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The metamorphic crystalline core of the Himalaya is classically considered to be composed of a single tectonic unit, the High Himalayan Crystalline Sequence (HHCS), thrusted during Early Miocene over the low-grade sediments of the Lesser Himalaya along the Main Central Thrust (MCT ). However, recent geochronological data from the MCT footwall (e.g., Catlos et al. 2002) indicate high-temperature protracted tectono-thermal activity up to Pliocene in the frontal part of the orogen, from Eastern Nepal to Garhwal (India). Further to the west, in the Sutlej Valley area (Himachal Pradesh, India), the metamorphic crystalline core is composed of two lithotectonic units, the High Himalayan Crystalline Sequence and the Lesser Himalayan Crystalline Sequence (LHCS). The latter crops out within a largescale antiformal tectonic window called the Larji-Kullu-Rampur Window. Both units show inverted metamorphic field gradients. New oxygen isotope thermometry combined with multiple equilibrium thermobarometry constrains the temperature and pressure field gradients for the LHCS. The 2.5 km thick, mylonitic schists forming the lowermost part of the unit show inverted temperature and pressure profiles, increasing upsection from about 520 °C at 6.0 kbars to about 600 °C at 8.6 kbars. These data, together with the ubiquitous top-to-the-south sense of shear indicators associated to the mylonitic deformation, suggest that the metamorphic isograds were passively sheared and inverted during the extrusion of the unit. Above the mylonitic zone of the LHCS, the temperature profile flattens at about 620 to 650 °C, whereas the pressure shows a marked decrease from about 8.6 to 6.5 kbars. These data are consistent with two different scenarios: 1) After having reached a maximum burial depth of about 30 km, the LHCS unit initially experienced a temperature increase during the beginning of its extrusion, as a consequence of the relaxation of the deflected isotherms. As a function of their position in the unit, mineral assemblages recorded conditions corresponding to different periods of the thrusting/extrusion history. 2) Peak metamorphic conditions were reached contemporaneously in the whole unit and peak isotherms were strongly inverted. Both pressure and temperature gradients were subsequently deformed by a heterogeneous simple shear flow such as proposed in the channel flow model by Grujic et al. (1996). A combination of the two proposed scenarios is conceivable as they are not mutually exclusive. FIGURE 1. (a) Monazite crystallization ages and muscovite cooling ages from the Himalayan crystalline core in the Sutlej Valley, showing the distinct evolutions of both units. The monazite crystallization ages indicate a temperature peak after 6.4 Ma in the LHCS and are in agreement with a temperature peak after ~ 23 Ma in the HHCS. (b) Cooling history of the LHCS unit, characterized by an average cooling rate of about 100 °C/Ma, which corresponds to an extrusion rate of about 8 mm/year along the Munsiari Thrust.

Renewed tectonic extrusion of high-grade metamorphic rocks in the MCT footwall since Late Miocene (Sutlej Valley, India)