Regional geological setting

The Himalayan orogen, resulting from the continent-continent collision between the Indian and Eurasian plates began approximately around 55-50 million years ago (e. g. Le Fort, 1975, 1996; Rowley, 1996; Leech et al., 2005), is commonly subdivided into four longitudinal tectonostratigraphic domains, separated by major north-dipping tectonic contacts (Fig. 1). From south to north, and from lower to upper structural levels, these domains are the Sub-Himalaya, the Lesser Himalayan Sequence (LHS), the Greater Himalayan Sequence (GHS) and the Tibetan Sedimentary Series (TSS). The Sub-Himalaya domain, or Siwalik Sequence, consists of un-metamorphosed foreland deposits dated as Neogene. To the north, these deposits are bounded by the Main Boundary Thrust (MBT), along which they are thrusted by the LHS. The LHS consists of low-grade metasediments (metapelitic schists, carbonates and quartzites) associated with granitic orthogneiss (e.g. Upreti, 1999; Goscombe et al., 2006; McQuarrie et al., 2008, Khon et al., 2010 and references therein).

The GHS consists of medium-grade to anatectic rocks resting between the Main Central Thrust (as originally defined by Gansser, 1964) and the extensional South Tibetan Detachment System (STDS; Burchfield et al., 1992; Carosi et al., 1998; Kellet et al., 2010). The GHS consists of two main portions characterized by different rocks, here reported as lower and upper GHS (Fig. 2).

The lower structural levels of the GHS, namely the lower GHS (GHS-L), are composed of medium- to high grade metasediments and granitic orthogneisses, recording a 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; Mosca et al., 2012). These rocks define an inverted metamorphic sequence, roughly centered on the Main Central Thrust Zone (MCTZ; Goscombe et al., 2006, Mosca et al. 2012, 2013): this is the shear zone driving the juxtaposition of the high grade upper GHS over the LHS metasediments (see Searle et al. 2008 for an exhaustive discussion about the MCTZ). The upper GHS (GHS-U) consists of high-grade para- and orthogneiss, typically anatectic, also known as Higher Himalayan Crystallines (HHC). These rocks host networks and lens-shaped bodies of two-micas and tourmaline-bearing leucogranites, 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, 2013).

The Tibetan Sedimentary Series overlie the GHS along the STDS and consist of Upper Precambrian to Eocene sediments originally deposited onto the Indian continental margin (e.g. Gaetani and Garzanti, 1991).