300 MW Baspa II - India's largest private hydroelectric facility on top of a rock avalanche-dammed palaeo-lake (NW Himalaya): Regional geology, tectonic setting and seismicity

摘要

This study aims for the characterization of the geological setting of 300 MW Baspa II, India's largest private hydroelectric facility which was built on top of a relict rock avalanche dammed palaeo-lake (Baspa Valley, NW Himalaya). Geologically, the hydroelectric installation is located in the Higher Himalayan Crystalline, just above the active Karcham Normal Fault, which is reactivating the Early Miocene Main Central Thrust, one of the principal Himalayan faults. The area is seismically active and mass-movements are common. At ca. 8200 yr BP the Baspa River was blocked behind a 142 x 106 m3 rock avalanche dam which created a ca. 260 m deep palaeo-lake. The whole palaeo-lake was completely filled with sediments in about 3100 years, making the Sangla palaeo-lake to a very rare example of a mass-movement dam with very long duration. The hydroelectric installation was built with its intake situated directly on top of the mass-movement dammed palaeo-lake of Sangla, utilizing the convex knick point in the river profile to increase the head for Baspa II for ca. 125 m, compared to the reconstructed longitudinal profile of the river prior to the mass-movement, which amounts for about 18% of its design head. At least 5 levels of soft-sediment deformation have been recorded in the exposed part of the lacustrine sediments of Sangla palaeo-lake, including brecciated laminae, overturned laminae, folds, faults and deformation bands, separated by undeformed deposits. They are interpreted as seismites, indicating at least 5 earthquakes within 2495 ± 297 years strong enough to cause liquefaction. These observations extend the local seismicity record considerably into the past and indicate more and possibly stronger seismic events than might be expected from the instrumental measurements.