Soil Gas Profiling as a Tool to Delineate Some Active Faults in NW Himalayas, India

English Abstract:

The state of Himachal Pradesh is considered to be seismically very active because as per Seismic Zonation Map of India, most of its area falls in two seismic zones, i.e. Very High Damage Risk Zone, Zone V & and High Damage Risk Zone, Zone IV. The lesser Himalayan tectogen lies mainly on the southern part of Himachal Pradesh state and is bounded between Main Central Thrust (MCT) and Main Boundary Thrust (MBT). Besides the longitudinal lineaments several transverse lineaments occur as faults and fractures trending normally or obliquely to Himalayan trend. In an effort to signify the role of radon and helium as a productive tool to delineate some active faults and lineaments, measurements were made in the soil-gas along some of the major thrust (MBT, MCT) areas of Himachal Himalayas.

Remote sensing data provides the synoptic coverage of any desired area and has been successfully used to recognize structures having tectonic significance. As a step in identification of active faulting and structural investigation, we will discuss the specific geochemical studies applied with aim of further identifying active faults and also complementing and specifying remotely sensed structures and zones. This method to investigate active tectonic structures, using soil gas composition at faults, provides relevant information about regional stress conditions, which can be obtained rapidly and at relatively low cost. Elevated emanation of radon and helium gases were detected over some of the thrust/faults/lineaments, thus indicating anomalous permeability of these zones in comparison with the adjacent areas. The collected soil gas samples were analyzed for radon and helium using RTM-2100 and Helium leak detector respectively. It can be concluded from present study that soil gas radon and helium patterns, combined with morphological and geological observations, can supply useful constraints for deformation tectonic environments. These findings may have important connotations for the long-term seismic hazard assessment of the tectonically active regions of the NW Himalaya. This methodology is an inexpensive method of locating unknown faults. Consequently, our work is a preliminary study in this region and the obtained results will considerably help in constructing the radon and helium map of the faults system in the NW Himalayas, India.