Lithology of Higher Himalaya and Tethys zone

Higher (Great) Himalaya. Lies between the MCT and STDS; altitude about 4000-8848 m. Rocks are Precambrian to Early Palaeozoic high-grade metamorphic rocks and granites.

Lithology (three formations)

• Formation I (lower): kyanite-sillimanite schist and gneiss, with migmatite.
• Formation II (middle): calc-silicate gneiss, marble and diopside-bearing rocks.
• Formation III (upper): augen gneiss, quartzite, and the Manaslu and other leucogranite intrusions.

Structure: the Higher Himalaya is a thick slab of crystalline rocks whose foliation dips generally northward at gentle to moderate angles. It is bounded by the MCT below and the STDS above, and is folded and sheared near these boundaries. Glacial and periglacial landforms are common.

Engineering problems: very high relief, rockfall, avalanches, glacial lake outburst floods (GLOF), glacial and moraine hazards, hard rock needing blasting, and difficult access.

Tibetan-Tethys (Inner, Trans) Himalaya. Lies north of the Higher Himalaya, beyond the STDS; altitude about 2500-5000 m. It has a dry, rain-shadow climate.

Lithology: Cambrian to Cretaceous (and Eocene) marine sedimentary rocks, rich in fossils: limestone, shale, sandstone, slate, quartzite and marl; the Thakkhola graben has Neogene sediments. The famous ammonite fossils (shaligram) of the Kali Gandaki valley come from Jurassic shale.

Structure: a thick sedimentary pile, unmetamorphosed or weakly metamorphosed, folded (open to tight folds) and thrust in places, and cut by later normal faults (Thakkhola graben).

Engineering problems: karst in limestone, frost action, permafrost, rockfall, and difficult access.