Active tectonics in Eastern Lunana (NW Bhutan): Implications for the seismic and glacial hazard potential of the Bhutan Himalaya

Abstract

Paleoseismological investigations, brittle fault analysis, and paleostrain calculations combined with the interpretation of satellite imagery and flood wave modeling were used to investigate the seismic and associated glacial hazard potential in Eastern Lunana, a remote area in NW Bhutan. Seismically induced liquefaction features, cracked pebbles, and a surface rupture of about 6.8 km length constrain the occurrence of M ≥ 6 earthquakes within this high‐altitude periglacial environment, which are the strongest earthquakes ever been reported for the Kingdom of Bhutan. Seismicity occurs along conjugate sets of faults trending NE‐SW to NNW‐SSE by strike‐slip and normal faulting mechanism indicating E‐W extension and N‐S shortening. The strain field for these conjugate sets of active faults is consistent with widespread observations of young E‐W expansion throughout southern Tibet and the north Himalaya. We expect, however, that N‐S trending active strike‐slip faults may even reach much farther to the south, at least into southern Bhutan. Numerous glacial lakes exist in the investigation area, and today more than 100 × 106 m3 of water are stored in moraine‐dammed and supraglacial lakes which are crosscut by active faults. Strong earthquakes may trigger glacial lake outburst floods, and the impact of such flash floods may be worst 80 km downstream where the valley is broad and densely populated. Consequently, tectonic models of active deformation have to be closely linked with glacial hazard evaluation and require rethinking and modification.

Active structures of the Himalayan-Tibetan orogen and their relationships to earthquake distribution, contemporary strain field, and Cenozoic volcanism

“…Spatially, seismic gaps along the Himalayan arc can be as long as 800 km (Avouac, 2007). It has been argued that strike-slip faulting is more active in the High Himalaya than previously recognized (Murphy and Copeland, 2005;Li and Yin, 2008;Meyer et al, 2006;Velasco et al, 2007).…”

Geologic correlation of the Himalayan orogen and Indian craton: Part 2. Structural geology, geochronology, and tectonic evolution of the Eastern Himalaya

“…1). In Bhutan, the work of Jangpangi (1974) and Gansser (1983) laid a foundation for the general geology that led to prolifi c studies across the country in the past three decades (Ray et al, 1989;Swapp and Hollister, 1991;Ray, 1995;Bhargava, 1995;Edwards et al, 1996;Grujic et al, 1996Grujic et al, , 2002Grujic et al, , 2006Davidson et al, 1997;Stüwe and Foster, 2001;Wiesmayr et al, 2002;Daniel et al, 2003;Tangri et al, 2003;Baillie and Norbu, 2004;Carosi et al, 2006;Meyer et al, 2006;Richards et al, 2006;Drukpa et al, 2006;Hollister and Grujic, 2006;McQuarrie et al, 2008). Following the traditional defi nition of major Himalayan structures and lithologic units by Heim and Gansser (1939), the Bhutan Himalaya is divided into the Lesser Himalayan Sequence, Greater Himalayan Crystalline Complex, and Tethyan Himalayan Sequence units bounded by the Main Boundary thrust below, the Main Central thrust in the middle, and the later discovered South Tibet detachment at the top ( Fig.…”

Geometry and kinematics of the Main Himalayan Thrust and Neogene crustal exhumation in the Bhutanese Himalaya derived from inversion of multithermochronologic data

“…Preserved at the base of klippen, the Outer South Tibetan Detachment shear zone (O‐STD) [ Kellett et al ., ] is characterized by top‐down‐to‐the‐north shearing with a strong component of vertical shortening and was active between circa 24–22 Ma and circa 16 Ma [ Chambers et al ., ; Corrie et al ., ; Grujic et al ., ; Kellett et al ., , ; Tobgay et al ., ]. Straddling the Bhutan‐Tibet border, the Inner South Tibetan Detachment (I‐STD) was active until circa 11 Ma as a ductile shear zone [ Edwards et al ., , ; Kellett et al ., ; Wu et al ., ] and is crosscut by steep brittle normal and strike‐slip faults of minor magnitude, which offset Quaternary moraines [ Meyer et al ., ; Wiesmayr et al ., ].…”