Discussions and conclusions
The structural architecture of the Himalayas in the investigated sector of far eastern Nepal is dominated at the regional-scale by the consistent dip to the north of tectonic boundaries and composite foliations, parallel to lithological contacts and compositional banding (Fig. 2). However, this apparently simple structural setting is the result of a high-strain and chronologically protracted deformational history, which was actually recorded in all the mapped tectonostratigraphic units by widespread transposition of earlier planar fabrics, and development of shear zones and mylonites.
As widely documented in other sectors of the Himalayan chain, also in the investigated sector of far eastern Nepal the southward extrusion and juxtaposition of the high-grade mid-crustal HHC over the low- to medium-grade metasedimentary LHS rocks is marked by the MCTZ, a north-dipping ductile to brittle-ductile shear zone 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 area (see profiles in Fig. 2; Mosca et al., 2011). Considering the results of original field mapping and meso- and micro-structural data, the MCTZ has been here identified as a zone of “high strain” interesting both LHS and GHS and only roughly centred on the IMS, in which different lithologies are more pervasively deformed and sheared with respect to adjacent rocks. Neither a single thrust nor a set of adjacent, minor thrusts, have been identified and mapped in the field to clearly define the boundaries of the MCTZ. On the contrary, the lower ductile boundary of the MCTZ is marked by the occurrence of phyllonites and mylonitic schists in the uppermost portions of the LHS, immediately below the contact with strongly mylonitic augen-gneisses. The upper ductile boundary of the MCTZ is located at the base of the “Barun-type” gneiss (i.e. the lower portion of the HHC), and is characterized by evidence of pervasive ductile deformation and boudinage. Across the MCTZ and its adjacent domains the abundant kinematic indicators and pervasive stretching lineations mark an uniform top-to-south sense of regional thrusting.
Micro- and meso-structural data show that the distribution and the intensity of deformation is heterogeneous within the MCTZ, thus suggesting a partitioning of deformation into different deformation domains as usually observed within shear zones at different scales. This is also partly favoured by difference in lithological compositions, which results in contrasting rheology of the sheared rocks. Moreover, structural data combined with petrologic results clearly indicate that the MCTZ is internally imbricated, resulting in the juxtaposition of rock packages characterized by different P-T evolution and T/depth gradients. This is particular evident in Fig. 13, in which the peak P-T conditions recorded by the studied samples and the correspondent T/depth gradients are reported as a function of their structural level (i.e. present geometrical position). Fig. 13 shows that, although the peak-T appears to continuously increase from the lower to the upper structural levels of the MCTZ, three different rock packages – separated by “metamorphic discontinuities” – may be distinguished on the base of their T/depth gradients: (i) ~ 21°C/km for the LHS phyllites and schists; (ii) ~ 20°C/km for the lower IMS micaschists and gneisses, and (iii) ~ 25°C/km for the upper IMS micaschists and anatectic gneisses. These discontinuities revealed by thermobarometric results do not always correspond to structural discontinuities clearly evident on the field (i.e. single thrust or shear zones), being the whole MCTZ (as intended and presented above in this paper and in Mosca et al., 2011) a zone highly strained: for this reason they are called “metamorphic discontinuities” (see also Groppo et al., 2009 and Yakymchuk and Godin, 2012 for a similar use of this term). The lowermost metamorphic discontinuity (dashed line in Fig. 13) is approximately located at the lithological contact between the LHS phyllites and the IMS mylonitic augen gneisses, thus roughly coinciding with the MCT as originally defined by Heim and Gansser (1939) and Goscombe et al. (2006). On the contrary, the uppermost metamorphic discontinuity (dotted line in Fig. 13) is located within the IMS and does not coincide with any “conventionally” recognized major structure. The upper ductile boundary of the MCTZ, located structurally above the uppermost metamorphic discontinuity discussed in this paper (i.e. at the base of the “Barun-type” gneiss, possibly the equivalent of the HHT of Goscombe et al., 2006), also represents an important metamorphic discontinuity.
Within the MCTZ, very similar results were obtained by Groppo et al. (2009) about 50 km westward, along the Milke Danda transect on the eastern side of the Arun tectonic window. In particular, peak P-T conditions estimated using the same method (i.e. “Average PT”) are highly comparable between the two adjacent areas (Fig. 13), as well as are the qualitative P-T trajectories inferred for the studied samples on the base of mineral chemical data, garnet zoning and microstructural observations. This observation allows to speculate that the geometry of the P-T paths quantitatively very well constrained by Groppo et al. (2009) westward, could then be reasonably extended also eastward to the studied samples (Fig. 12b).
The results here obtained for the Kanchenjunga area confirm the existence of an important metamorphic discontinuity within the MCTZ (i.e. well above the MCT as originally defined by Heim and Gansser, 1939, but below the HHT of Goscombe et al., 2006). This metamorphic discontinuity juxtaposes medium- to high-grade (locally anatectic) rocks that experienced moderate peak-P conditions, onto medium-grade rocks that experienced higher peak-P conditions. The existence of this discontinuity, firstly documented by Groppo et al. (2009) but already suggested by the P-T data presented by Goscombe et al. (2006) and Imayama et al. (2009) (Fig. 13), further supports the imbricated nature of the MCTZ (see also e.g. Yakymchuk and Godin, 2012) and suggests that the detailed comprehension of such a complex ductile shear zone can be only achieved by integrating structural, petrologic and geochronologic studies at different scales.