Lesser Himalayan Sequence

The LHS, which is also referred to as the Midlands Group in central Nepal (Upreti, 1996), includes dolomite, limestone, impure quartzite, marble, phyllite, schist, orthogneisses and metabasaltic rocks. These rocks have been affected by greenschist to lower amphibolite facies metamorphism (Upreti 1999; Hodges, 2000). The LHS can be subdivided in two units interpreted to be separated by an unconformity (Upreti, 1999 with references). The lower unit or the “Lower Lesser Himalaya”, comprises sedimentary-volcaniclastic rocks and orthogneisses ranging in age from Paleo-proterozoic to Meso-Proterozoic (Upreti, 1999 and references therein). The upper unit, the “Upper Lesser Himalaya”, dominantly comprises carbonaceous phyllite and marble, associated quartzites, and graphitic rich rocks with middle Proterozoic protoliths.

The distinction between rocks of the GHS and rocks of the LHS is largely associated with the mapped location of the MCT. In this field guide we map the MCT approximately at the northern end of village of Dana (Fig. 3) and thus the rocks that crop out to the south in the interpreted footwall comprise the LHS. The LHS in the Kali Gandaki valley comprises an almost complete section beginning with the early Proterozoic Kuncha Formation at the lowest levels exposed and extending up to the middle Proterozoic Robang Formation (Larson and Godin, 2009). The Kuncha Formation comprises the bulk of the Lower Lesser Himalaya in the field trip area; it is a >5 km thick package of phyllitic schist and meta-sandstone. Conspicuously absent in the lower section along Kali Gandaki valley is the Paleoproterozic (DeCelles et al., 2000) Ulleri augen orthogneiss, a km-scale thick band of mylonitized potassium feldspar augen-bearing unit. This orthogneiss is well exposed at Ulleri village, south-east (about 3 km) of Deurali-Ghorepani village (Pêcher and Le Fort, 1986). The Ulleri, which has been interpreted as a syndepositional intrusion (Pêcher and Le Fort, 1977), occurs in the Kuncha Formation in adjacent valleys to the east (Colchen et al., 1986) and its equivalents are mapped across much of the central Himalaya.

The base of the Upper Lesser Himalaya in the study area is marked by quartzite of the Fagfog Formation (also referred to locally as the Ghandrung quartzite). The tectono-stratigraphy above the Fagfog Formation comprises a succession of alternating carbonaceous phyllitic schist and marble or dolostone units. These include from lower to higher position: (1) the interbedded carbonate rocks and phyllitic carbonaceous schist of the Dandagaon and Nourpul Formations, which cannot be separated in the Kali Gandadki, (2) the metadolostone and marble of the Dhading Formation, (3) the phyllitic schist and carbonaceous schist of the Benighat Formation with intercalated Jhiku carbonate beds, and (4) the conspicuous marble of the Malekhu Formation which is overlain by non-calcarous phyllitic schist (Larson and Godin, 2009).

Yoshida et al. (2005) and Rai et al. (2005) observed a series of highly sheared rocks (e. g. phyllonite, mylonite-ultramylonite and mylonitic granite, garnet-bearing gneiss and schist (total thickness about 700 m) between Duwari Khola to the south and the MCT, north of Dana. Field observation shows a sudden change in both lithology and metamorphic grade of these rocks compared with the underlying uppermost LHS rocks. This unit is interpreted to represent a higher grade metamorphic equivalent of the Lower LHS (Kuncha Formation and Ulleri type augen gneiss exposed at Ulleri village, south of Deurali-Ghorepani village). Therefore, a thrust locally named as the Duwari Khola Thrust (DKT) is placed at the contact between the mylonitic rocks in the north and the black schist of the upper LHS in the south. This 700 m thick sheared zone is considered as the MCT zone.