Geologic Mapping of the South Tibetan Detachment System in Western Bhutan Using ASTER Multispectral Data

Remotely sensed data is a powerful tool for studying geology in areas where traditional field geology proves costly or difficult. This study focuses on geologic mapping of the South Tibetan Detachment System (STDS) in the area where Bhutan, India, and China meet. Having improved geologic maps is important as background information for better understanding the evolution and structure of the regional geology. Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) multispectral imagery was used to differentiate the low-grade metamorphosed Tethyan Sedimentary Sequence (TSS) of the hanging wall from the mid- to high-grade metamorphosed Greater Himalayan Sequence (GHS) of the footwall. The TSS contains distinct carbonate beds that differ greatly from the granitic GHS. These spectral differences were used to create a false color image that clearly shows the extent of each unit in the area. A method called forced invariance processing was also used to diminish the spectral influence of patchy areas of glaciers, melt water, snow, and vegetation. After the extents of the units were revealed, mapping was done based on the known geometries of the fault, high-resolution photo interpretation, and topography. The results of this study show that previous mapping of the area has underestimated the extent of the TSS. TSS units were found 1-3 km farther south than previously shown. Further exploration of this area would be needed to interpret whether this is a structural phenomenon or, more likely, a difference in defining the metasedimenatry unit that lies between the clear boundaries of the GHS and TSS