Regional tectonic setting
The Himalayan orogen is the result of the continent-continent collision between the Indian and Eurasian plates that began approximately around 55-50 million years ago and still continues today (e.g. Le Fort, 1996; Rowley, 1996; Guillot et al., 2003; Leech et al., 2005). Four main longitudinal tectonostratigraphic domains are usually distinguished along strike, bounded by north-dipping major tectonic discontinuities. From south to north, and from lower to upper structural levels, these are (Fig. 1) the Sub-Himalaya, the Lesser Himalayan Sequence (LHS), the Greater Himalayan Sequence (GHS) and the Tibetan Sedimentary Series (TSS).
The Sub-Himalaya domain consists of un-metamorphosed syn-orogenic sediments (sandstones, shales and conglomerates, also reported as “Siwalik group” in the literature) dated as Neogene and deposited on foreland basins. To the north, these deposits are bounded by the Main Boundary Thrust, along which they are thrusted by the LHS. The Lesser Himalayan Sequence consists mainly of low-grade metasediments (metapelitic schists and quarzites) associated with granitic orthogneiss (see for instance Upreti, 1999; Goscombe et al., 2006; McQuarrie et al., 2008, Khon et al., 2010 and references therein). The LHS is subdivided into a number of stratigraphic and/or fault bounded units (e.g. Upreti, 1999; Paudel and Arita, 2000) and at a regional scale it forms a duplex structure (e.g. De Celles et al., 1998; Mitra et al., 2010). The LHS is bounded at its top to the north (Fig. 1) by the Main Central Thrust (MCT as originally defined by Gansser, 1964) that separates the LHS from the overlying GHS.
The Greater Himalayan Sequence, bounded to the north by the South Tibetan Detachment System (STDS; Burchfield et al., 1992; Carosi et al., 1998; Kellet et al., 2010), consists from the lower to the upper structural levels of: (i) medium- to high grade metasediments and granitic orthogneisses roughly centred on the Inverted Metamorphic Sequence (IMS), its metamorphic grade increasing structurally upward from the staurolite zone to the sillimanite zone and, locally, to anatexis (e.g. Goscombe et al., 2006; Groppo et al., 2009, 2010). According to some authors the IMS is bounded at its top by a structural discontinuity (MCT of Bordet, 1961; High Himal Thrust – HHT - of Goscombe et al., 2006, see Fig. 1 and the following discussion); (ii) high-grade para- and orto-gneisses, often anatectic, hosting networks and lens-shaped bodies of two-micas and tourmaline-bearing leucogranites, which can be as thick as 1-2 km at Mts. Makalu and Baruntse (Visonà and Lombardo, 2002) and up to 8 km thick at the Manaslu in Central Nepal (Guillot and LeFort, 1996). These high-grade gneisses are known as Higher Himalayan Crystallines (HHC) (Fig. 1) and are characterized by a progressive decrease in peak-pressure structurally upward (Pognante and Benna, 1993; Lombardo et al., 1993; Davidson et al., 1997; Guillot, 1999; Hodges, 2000; Groppo et al., 2012).
The Tibetan Sedimentary Series overlie the HHC along the South Tibetan Detachment System and consist of Upper Precambrian to Eocene sediments originally deposited onto the Indian continental margin (Gaetani and Garzanti, 1991).