THE USE OF LICHENOMETRY FOR ASSESSMENT OF THE DESTRUCTION AND RECONSTRUCTION OF BUDDHIST SACRED WALLS IN LANGTANG VALLEY, NEPAL HIMALAYA, FOLLOWING THE 2015 GORKHA EARTHQUAKE

Mani walls, Buddhist sacred walls constructed of carved blocks with Tibetan letters and elaborate imagery, are common in Langtang Valley, Nepal Himalaya. Fieldwork in 2009-2014 involved mapping and photographing all 80 mani walls, measuring and photographing all occurrences of the crustose lichen Rhizocarpon geographicum, and interviewing local informants regarding the history and traditions of the mani walls. The consensus of the informants was that the mani walls were constructed 400-600 years ago and that the original mani wall was in the village of Ghoratabela at the mouth of the glacial valley. An apparent lichen growth curve was developed using five sources of indirect data, including the foundation of one stupa (sacred monument) and two locations of former ice cover, for which ages were obtained from local informants, and two debris ridges that had been dated by ¹⁰Be. Based on the growth curve, the oldest lichen on a mani wall dated only to 1942, which, within measurement error, was concurrent with the 1934 earthquake, the last major earthquake in Nepal prior to the Gorkha earthquake of April 25, 2015. An icefall-debris avalanche triggered by the 2015 earthquake completely buried Langtang Village, killing at least 350 people. In November 2015 all previous fieldwork was repeated, including additional interviews. It was found that 15% of mani walls could not be located and 20% were severely damaged (it was not obvious how to reassemble the scattered blocks). All other mani walls were fully intact (there were no slightly damaged walls). The original mani wall had apparently been reconstructed 170 m from its previous location. In two severely damaged and in three fully intact mani walls, large lichens (12-49 mm) were found that were not present during previous fieldwork. These previously unseen lichens had strong color contrast from white to pale yellow, as opposed to the bright yellowish-green of healthy R. geographicum. The most likely explanation was that the three intact mani walls had already been reconstructed using previously interior blocks as exterior blocks. This research raises the possibility that (1) many Himalayan religious structures are not the original structures, but are replicates that are reconstructed after natural disasters (2) Himalayan religious structures could be used to date natural disasters.