Greater Himalayan Sequence

The metamorphic core of the Himalaya is represented by the GHS, a continuous belt of high-grade metasedimentary and meta-igneous rocks with associated Miocene leucogranites (Carosi et al., 1999; Visonà & Lombardo 2002; Upreti, 1999; Hodges 2000; Visonà et al., 2012). This litho-tectonic unit typically forms the central part of the belt and is often associated with the highest topographic relief. The GHS occurs in the footwall of the STDS and is itself over-thrust atop the LHS along the Main Central thrust (MCT). The MCT is a controversial tectonic feature and has variably been described as a wide zone (up to 4-5 km some sections) of deformation is characterized by mylonites locally overprinted by cataclasites (Carosi et al., 2002, 2007), a structure associated with the kyanite isograd (e.g. Le Fort, 1975) and an inverted field metamorphic gradient, a break in Nd isotoptes (e.g. Robinson et al., 2001), a break in U-Pb detrital zircon ages (e.g. Parrish and Hodges 1996), or a strain gradient at the base of rocks metamorphosed and cooled during the Miocene evolution of the mountain belt (Searle et al., 2008). The main foliation strikes NW-SE, dips moderately to the NE with down-dip object lineations and typically records a top-to-the SW sense of shear (Colchen et al., 1986; Hodges, 2000). In this guide we put the MCT just above Dana village, accordingly to the original definition of Le Fort (1975) and Colchen (1986) which separates the kyanite and garnet bearing gneiss of the GHS from the lower quartzites of the LHS.

Le Fort, (1975) recognized three formations, which were later re-considered as tectonic units (Searle and Godin, 2003).

- Unit 1. The base of the GHS consists of predominantly clastic metasedimentary rocks, represented by biotite-muscovite-garnet-kyanite gneisses, although mica schists and phyllites, calc-schists, quartzites, para-amphibolites, and subordinate impure marbles are also present. Layering in the unit dips moderately northward in most exposures (Vannay and Hodges, 1996; Hodges, 2000; Rai et al., 2005). Unit 1 has been traditionally considered as a uniform crustal section with a variable thickness from 1 km to more than 20 km along strike (Le Fort, 1975).

- Unit 2. Unit 1 is overlain by a 2-4 km thick sequence of amphibolites-facies, banded calc-silicate gneiss, paragneiss, marble and amphibolite. The boundary between units 1 and 2 is is parallel to the compositional layers in both packages of rocks. The lack of recognized tectonic or metamorphic discontinuities at the transition between the two units and the absence of localized tectonic fabrics, has been originally interpreted by some authors (Colchen et al., 1986) as an evidence to consider the two units as an unique block. The transition between the two units is gradual and highlighted by changes in mineral composition.

- Unit 3. The unit 3 comprises nearly homogeneous Cambrian-Ordovician orthogneiss (Pognante et al., 1990; Godin et al., 2001) that is intruded by a network of Miocene sills and leucogranitic dykes. The intrusions may be related to the 4-5 km thick Manaslu leucogranite present few 10s of kilometers to the east, one of the larger granitic body within the Himalaya (Searle, 2010). The orthogneiss is disrupted by the ductile Kalopani Shear Zone (KSZ) (Vannay and Hodges, 1996; Godin et al., \1999b; Godin et al., 2001; Godin et al., 2003; Searle, 2010) (Fig. 5), which is characterized by highly strained orthogneiss and migmatitic gneiss. Isotopic Rb-Sr data indicate that the unit 3 protolith is Cambrian-Ordovician (Pognante et al., 1990), this is in agreement with U-Pb zircon ages (Godin et al., 2001).

The uppermost part of unit 3 is marked by the Annapurna Detachment (Figs. 5, 8), a shear zone that juxtaposes the orthogneiss of Unit 3 with rocks of the Largjung Formation. The Largjung Formation is characterized by poly-deformed metapelites and marbles. Colchen et al. (1986) considered these rocks as part of the TSS. Other studies, however, have included the Larjung Formation as part of the Unit 3 of the GHS (Vannay and Hodges, 1996). A similar situation is described in the adjacent Modi Khola valley and in the Marsyangdi valley between Annapurna and Manaslu massifs farther to the east (Figs. 1, 3), where calc-silicate rocks are mapped structurally above and below the orthogneiss of Unit 3. More recently Unit 3 is considered constituted by orthogneiss, metapelites, marbles and calc-silicates (Vannay and Hodges 1996; Godin et al., 2001; Searle, 2010).

The main fabric in the GHS is a pervasive, transposition foliation formed during a second deformation phase (S2; Carosi et al., 1999, 2007, 2010, in press). It is often recognizable as a shear band cleavage as defined by Passchier and Trouw (2005). The S2 foliation typically strikes NW‐SE and dips 30°–60° toward the NE. It is marked by the preferred orientation of metamorphic minerals and recrystallized quartz ribbons. Kyanite, staurolite, muscovite, and biotite are occasionally bent or kinked along shear bands. Top‐to‐the‐SW sense of shear is marked by C‐S fabric, shear bands, asymmetric tails around porphyroclasts, and rotated garnets within the mylonites in the lower portion of the GHS. The elongation lineation (L2) trends NE-SW and plunges NE moderately to steeply (20°, 60°). S1, formed during D1 deformation, is sometimes preserved as a relict in D2 fold hinges (F2) and S2 microlithons and as internal foliation in porphyroblasts (Carosi et al., 2010). The GHS underwent at least two later folding phases, characterized by nearly orthogonal NW‐SE and NE‐SW trending fold axes, resulting in kilometer‐scale open folds with steeply dipping axial planes. These folds, well-expressed in western Nepal, affect the tectonic boundaries (see geological cross sections in Upreti (1999) and Carosi et al. (2002, 2007) and have also been described eastward in the Mt. Everest‐Mt. Makalu region, Sikkim and Bhutan (Lombardo et al., 1993; Carosi et al., 1999; Schelling, 1992).