Shield-Like Lithosphere of the Lower Indus Basin Evaluated from Observations of Surface-Wave Dispersion by Srichand Prajapati, Mukesh Chauhan, Arun K. Gupta, Rashmi Pradhan, G. Suresh, and S. N. Bhattacharya Abstract The lithospheric velocity structure of the lower Indus basin has been evaluated through inversion of fundamental modes of both Love and Rayleigh wave group velocities from the broadband records of a seismic network maintained by the Institute of Seismological Research, Gujarat, India. We have considered three clusters of wave paths A, B, and C that mainly cross the lower Indus basin from south to north; the wave paths of A mainly cross the continental shelf, and the wave paths of B and C pass through the lower Indus basin. The measured group velocities correspond to periods of 5 to 90 s for Rayleigh waves, and 5 to 115 s for Love waves. These data sets resolve the structure of the lithosphere through a nonlinear inversion based on a genetic algorithm with a wide solution space. The mean and standard deviation (S.D.) of the 70 accepted solutions for each of these three clusters provide the 2D struc- ture for the lower Indus basin from south to north. The sediment consists of two layers with total thickness from 5.7 to 6.6 km increasing northward. The crustal thickness also increases northward from 32.9 (cluster A) to 39.7 km (cluster C) in the lower Indus region. The S-wave velocity below the crust varies from 4.55 to 4:59 km=s, which is close to the corresponding velocity of 4:60 km=s of the Indian shield region to the east of the Aravalli range. The thicknesses of the lithosphere, as well as the velocities of the uppermost mantle of the lower Indus plain, are similar to that of the Indian shield. Introduction The lithospheric structure of the Indian subcontinent inferred from surface-wave dispersion data has been studied by many workers (Bhattacharya, 1981, 1992; Singh, 1999; Mitra et al., 2006; Suresh et al., 2008; Bhattacharya et al., 2009). Bhattacharya (1992) reviewed the results of surface- wave dispersion in the Indian region and suggested litho- spheric models for the Indian shield and the Indo-Gangetic basin and found that the model for the shield extends to the east of the Aravalli range (Bhattacharya, 1991). It was also found that the upper mantle of the Indo-Gangetic basin is the same as that of the Indian shield. Suresh et al. (2008) studied the lithosphere of the Indus block in the northwest of the Indian subcontinent through surface-wave dispersion using data from the Muzaffarabad earthquake (M w 7.6) and its aftershocks on the western syntax of the main boundary fault. The wave paths are nearly parallel to the Aravalli trend and cross the eastern edge of the Indus rift bordering the Marwar craton and covering a part of the middle Indus basin (Balakrishnan et al., 2009). However, much less is known about the lithospheric structure for the lower Indus block. In this study, we have obtained the lithospheric velocity of structure of the lower Indus basin through inversion of both Love and Rayleigh wave group velocities. The wave paths cross the lower Indus block including Shagarah basin in the northwest of the Indian subcontinent (Fig. 1). Tectonic and Crustal Features Geodynamic changes in the northwestern Indian shield from the Jurassic to the Cretaceous–Tertiary boundary were in response to continental breakups (Sharma, 2005). The continental fragmentation under an extensional tectonic regime resulted in magmatism and Indus basin tectonism in the northwestern Indian shield during the Cretaceous– Tertiary period. The Indus basin developed as an extensional basin as a result of divergence of the Indian subcontinent from Gondwanaland (Zaigham and Mallick, 2000). The evolution of basins in western India began in the Mesozoic and continued into the Tertiary. The Jaisalmer basin, which is tectonically related to the Indus basin evolution, covers more than 30,000 square km in the northwestern Indian shield, extending as far as the Mari region of Pakistan. The basin is controlled by wrench-fault tectonics, and the basin fill indicates sedimentation from the Jurassic to the Tertiary (Sharma, 2007). Raja et al. (1989) and Kadri (1995) 859 Bulletin of the Seismological Society of America, Vol. 101, No. 2, pp. 859–865, April 2011, doi: 10.1785/0120100034