Introduction
The India-Asia convergence resulted in intense crustal shortening in the Himalaya since Paleocene (Searle et al., 1987; Zhu et al., 2005; Yin, 2006, Jain, 2014). It has mobilized the northern edge of the Indian Plate during the Cenozoic into a vast metamorphic belt including the metamorphic core of the belt, represented by the Higher Himalayan Crystallines (HHC). This belt is thrust southwards on to the sedimentary belt of the Proterozoic Lesser Himalayan Sequence (LHS) along the Main Central Thrust (MCT) (Heim and Gansser 1939; Le Fort 1975; Gansser, 1964; Hodges, 2000; Jain et al., 2002; Yin, 2006). The metamorphic core of the Himalayan Orogen in the Eastern Garhwal region of the NW India consists of nearly 20 km thick homoclinal sequence of amphibolite-granulite facies HHC (Fig. 1), and is covered by the late Precambrian to Eocene Tethyan Sedimentary zone (TSZ). The latter is bounded by the Indus-Tsangpo Suture zone (ITSZ) along its northern margin and the South Tibetan Detachment System (STDS) in the south (Burg et al., 1984; Burchfiel et al., 1992; Hodges, 2000).
The MCT with its zones of inverted metamorphic isograds from sillimanite grade down to biotite grade is one of the largest ductile shear zones known from any collision-related mountain belt. The MCT crops out along 2200 km length of the Himalaya from western Zanskar to Bhutan and Arunachal Pradesh (Searle et al., 2008; Jessup et al., 2008; Yin, 2006; Kellet et al., 2010). It dips north and places high-grade metamorphic rocks of the HHC south over unmetamorphosed or slightly metamorphosed rocks of the LHS all through its length. The HHC contains the most metamorphic rocks of the Himalayan orogen south of the the Indus-Tsangpo Suture zone, and therefore is a key to understand the metamorphic evolution and degree of exhumation within the orogen (Kohn et al., 2005). Exhumation of the HHC also in the Garhwal region is associated with southwestward directed thrust sense of shear along the MCT, focused erosion at the denudational front, and northeastward directed normal-sense shear along the STDS. As a consequence of this convergence, one of the most spectacular result was the melting of various rock types of the HHC and generation of migmatite and leucogranite, which have been recently modeled as midcontinental crustal channel flow extruding into the southward translating HHC (Beaumont et al., 2001; Godin et al., 2006; Grujic et al., 2002; Jamieson et al., 2004; Searle et al., 2006, 2010).
The STDS juxtaposes the unmetamorphosed TSZ down to the northeast over the high-grade metamorphic rocks of the HHC (Caby et al., 1983; Virdi, 1986; Patel et al., 1993; Gururajan and Choudhuri, 1999).The South Tibet Detachment System (STDS) was first recognized in southern Tibet within the contractional Himalayan orogen, having opposite sense of movement in contrast to the southward thrusting along the Main Central Thrust (MCT) (Burg et al., 1984; Burchfiel et al., 1992). Since then it has now been documented almost parallel to the Himalayan belt from Bhutan, Nepal and western Himalaya (Burchfiel and Royden, 1985, Herren, 1989; Burchfiel et al., 1992; Patel et al., 1993; Grujic et al., 1996; Hodges et al., 1996; Carosi et al.1998; Dezes et al., 1999; Jain and Patel, 1999; Jain et al., 1999; Godin et al., 2006). Initially, it was interpreted as a thrust by the Chinese researchers (Academia Sinica, 1979).
The STDS corresponds to a series of north-dipping structures accommodating top-to-the-north normal motion of the Tethyan Himalayan Sequence (THS) in southern Tibet with respect to the underlying Higher Himalayan Crystalline (HHC) belt (Burchfiel et al., 1992; Burg, 1983; Burg et al., 1984). Normal motion has occurred on several ~ parallel low-dipping structures along the STDS that reveal from top to bottom (i) few brittle normal faults in the THS, (ii) a detachment at the contact between the unmetamorphosed to poorly metamorphosed THS and the underlying metamorphic rocks, and (iii) a ductile shear zone at the top of the HHC, the STDS shear zone, having highly deformed gneisses with NE-trending lineation and numerous shear structures indicating a normal motion (Leloup et al., 2010).
The main objectives of this work are to document some typical characters of the Cenozoic India-Asia convergence in the Uttarakhand Himalaya along a cross-section along the Alaknanda-Dhauli Ganga Valleys in a 6-day field excursion:
1. Characters, position and definition of the MCT vis-a-vis the Munsiari Thrust (the MCT–I), and the Vaikrita Thrust (the MCT–II),
2. Position and characters of the South Tibetan Detachment System (STDS),
3. Deformation of the HHC, and its detailed shear sense analysis,
4. Structural control on melt accumulation of the Himalayan migmatites
5. Himalayan inverted metamorphism, and
6. Assessment of channel flow or other models that could try to explain the evolution of high grade metamorphic rocks and the processes involved in their exhumation.