AKR thanks Cambridge Commonwealth Trust for providing a scholarship to study at the University of Cambridge where this work formed a portion of his Ph.D. research. VKG thanks the Directors of IIA and C-MMACS for providing infrastructure support and the University of Cambridge for travel support. The authors thank Bob Hermann for providing computer programs used in this study and James Jackson for his valuable comments which greatly improved the text. All the figures were made with Generic Mapping Tools. The field program was supported by a Deep Continental Studies project of the Dept. of Science & Technology, N. Delhi.