Higher Himalayan Crystallines

The HHC represent the upper part of the GHS, bounded by the STD at its top. At the lower structural levels, the HHC consist of peculiar Grt + Kfs + Sil ± Ky anatectic paragneiss (Fig. 5a), with local intercalations of quarzite, impure marble and calc-silicate rock with the assemblage Di + Pl + Qtz ± Grt ± Amp (Fig. 5b, c). These gneisses (Rolwaling-Khumbu-Kangchenjunga Paragneiss in Schelling, 1992 and Jannu-Kangchenjunga Gneiss in Goscombe et al., 2006) are here considered to be the lateral equivalents of the Barun gneiss occurring in the Everest-Makalu region (Lombardo et al., 1993; Groppo et al., 2012).

Figure 5. Representative mesostructures of the HHC

Representative mesostructures of the HHC

(a) Grt + Kfs + Ky + Sil anatectic paragneiss (“Barun type”). SHHC foliation is defined by a peculiar compositional layering. (b) and (c) examples of boudins of calc-silicate granofels consisting of Pl + Grt + Di + Qtz within the “Barun type” paragneiss, stretched and enveloped by the main SHHC foliation. (d) Anatectic orthogneiss, with development of a ductile shear zone roughly parallel to the main SHHC foliation, in turn crosscut by a pegmatite dike. (e) Pegmatitic and aplitic dikes variably oriented with respect to the main SHHC foliation in the cordierite-bearing paragneiss. (f) Ductile shear zone developed in the upper levels of the HHC. a and c modified from Mosca et al., 2011.

The upper structural levels of the HHC consist of Bt + Sil + Crd ± Grt anatectic paragneisses, lacking kyanite and characterized by the occurrence of cordierite in significant amounts, and associated to large bodies of sillimanite-bearing anatectic orthogneisses (Fig. 5d). Pegmatitic dykes and leucogranite significantly increase towards higher structural levels (Fig. 2), variably oriented with respect to the main foliation (Fig. 5d, e).

It is generally difficult to give an univocal interpretation of all the penetrative structures and fabrics recognizable in the field in the high-grade lithologies of the HHC (Fig. 5), namely if they result from tectonic or from migmatization and/or melt-crystallization processes. At the outcrop scale, the HHC show a pervasive foliation usually parallel to the mesoscopic compositional layering and dipping at moderate angle towards the north and the east; this foliation has been generally reported and mapped as SHHC in the Fig. 2.

In the lower structural levels of the HHC (Fig. 5a), the SHHC foliation of the Barun-type gneiss is defined by millimetric to centimetric leucocratic domains rich in quartz and feldspar alternating with millimetric dark Bt + Pl + Sil layers. Garnet is always present and occurs as mm to cm large porphyroblasts generally enveloped by the Bt + Pl + Sil foliation, or within the leucocratic domains. A stretching mineral lineation LHHC, defined by aligned Sil, Qtz and Bt, is recognizable on the SHHC and plunges parallel to the SHHC dip. Dm to several m thick bodies of calc-silicate granofels (Fig. 5b, c), quarzites and marbles are observed stretched along very pervasive SHHC planes, often concentrated at the contact with the IMS schists and gneiss.

Moving structurally upwards, the SHHC identifies a pervasive foliation both in the Bt + Sil + Crd ± Grt anatectic paragneisses and in the Sil-bearing anatectic orthogneisses, respectively defined by Qtz + Feld + Bt + Sil + Cr, and Qtz + Feld + Bt + Sil assemblages. In the Sil-bearing anatectic orthogneisses, cm-scale K-feldspar porphyroclasts are stretched and rotated along the SHHC (Fig. 5d), and widespread S-C fabrics and rotational K-feldspar porphyroclasts define a top-to-south sense of shear.

Throughout the HHC, the SHHC is variably folded, displaced and deformed with irregular sigmoidal patterns in correspondence of, and within, ductile shear zones, ranging from a few cm up to a few m in thickness (Fig 5f). Most of these zones are at low to moderate angles with respect to the pervasive SHHC. Peculiar late shear zones, characterized by significant grain reduction and penetrative low-grade metamorphic foliation, have been identified in few outcrops at the lowermost structural levels (Mosca et al., 2011).

Field observation along the two studied transects suggest a significant increase of the mesoscale-folding of SHHC passing from the lower structural levels (Barun-type gneiss) towards the higher structural levels. In particular, the Crd-bearing anatectic gneiss is often deformed by folds exhibiting isoclinal geometry with thickened hinges and thinned limbs, and axial planes sub-horizontal or dipping toward the NW.

The Main Central Thrust zone in the Kanchenjunga area

According to a large number of field studies and interpretations from several sectors of the Himalaya, structural criteria are primary references to define and identify the MCTZ (see for instance Searle et al., 2009 and references therein for an exhaustive critical review).

Structural mapping in the Kanchenjunga area shows that, at the juxtaposition of the HHC over the LHS, the MCTZ identifies a ductile to ductile-brittle shear zone roughly centred on the IMS and showing minimum thickness on the order of 6-7 km in the north-western portion and 3-4 km in the south-eastern portion of the studied region (see transects of Fig. 2; Mosca et al. 2011). The boundaries of the MCTZ in the investigated areas cannot be traced along a single thrust or a set of adjacent discrete thrusts, but are recognized as zones of high strain, affecting both the upper portions of the LHS and the lower portions of the HHC (see also Mosca et al., 2011) where an increasing of the intensity of the shear is observed at the outcrop scale.

Following this rationale, the lower boundary of the MCTZ is progressively marked by the common occurrence of phyllonites and mylonitic schists in the uppermost portions of the LHS near to the contact with the strongly mylonitic augen-gneisses of the lower IMS. The upper boundary of the MCTZ is marked in the lower portion of the Barun-type gneiss by evidence of pervasive ductile shearing and boudinage. Throughout the whole MCTZ, a widespread pervasive development of mylonitic foliations is observed, parallel (at the map scale) to the main lithological contacts, as well as abundant kinematic indicators showing a consistent top to the south sense of shear.